General information about Routers and Routing

Router Memory :

Similar yet different from a regular computer, the router has different kinds of memory ROM, Flash, NVRAM, and SDRAM which all have different functions:

  • ROM – POST, Bootstrap, and ROMMON
  • Flash – IOS
  • NVRAM – Configuration File
  • SDRAM – Running-Config, Routing Table, IOS (everything is loaded and executed from RAM)

The router is a computer but it does not have a traditional hard drive to store files and the operating system, this is accomplished in Flash memory and NVRAM memory.

Router Bootup Process:

  1. POSTROM memory,
  2. BootstrapROM memory,
  3. Load the IOS – the router has an ordered routine for loading the IOS

    1. Flash Memory – the IOS is typically loaded from Flash memory
    2. TFTP – if there is no IOS in Flash, the router will search for a network TFTP server,
    3. ROM – if there is no IOS found, the router defaults to a recovery IOS called Rommon,
  4. Load the Startup-Configthe router has an ordered routine for loading the startup-config file

    1. NVRAM memory – the startup-config file is typically loaded from NVRAM memory
    2. TFTP – if there is no config file in NVRAM, the router will search for a network TFTP server,
    3. Setup-Mode – if there is no configuration file found, the router defaults to setup-mode

The Router’s Purpose:

The router’s purpose or function is to find the best path (route) and switch out of the correct interface. The router will make the decision of the “best path” by first determining the destination network, and second by consulting its routing table.

Static Routing and Dynamic Routing:

Static routing is a good choice for networks that: never change, are small in size or have only one router, or have only one way out of the network.

Dynamic routing is a good choice if a network has multiple routers, is part of a larger network, or if the network changes frequently.

For example, in a situation where the network changes, with a dynamic routing protocol if a network goes down, the routers will inform each other automatically through the routing protocol, and the route will be removed from the routing table; with static routing, if a network goes down, an administrator will have to go in and remove the the static route manually.

Different types of interior gateway routing protocols: RIPv1, RIPv2, EIGRP, and OSPF.

Routed Protocols
IPX/SPX (Novell – no longer in use)
Apple Talk (Apple – no longer in use)

Routing Protocols
RIP v1 – interior gateway protocol, IETF – RFC1058, open standard
RIP v2 – interior gateway protocol, IETF, open standard
EIGRP – interior gateway protocol, Cisco proprietary
OSPF – interior gateway protocol, IETF, open standard
ISIS – interior gateway protocol
BGP – exterior gateway protocol
Interior Gateway Routing Protocol Types
Distance Vector     Link State  



Thank you

Momataj Momo


Virtual local area networks (Vlans) Concepts

A VLAN is a group of logically network devices. such as a set of networked computers and printers for a department or building floor.and can seperate networks “guests” and trusted users traffic. A logically separate subnetwork which device on vlan 20 and Vlan 30 can not communicate without a layer 3 device.

The term VLAN stands for ‘Virtual LAN’ and Cisco defines a VLAN as a broadcast domain. Basically, what that means is that you can segregate certain ports on a single physical switch into logical switches (VLANs).VLAN’s allow a network manager to logically segment a LAN into different broadcast domains. Since this is a logical segmentation and not a physical one, workstations do not have to be physically located together. Users on different floors of the same building, or even in different buildings can now belong to the same LAN.VLAN’s also allow broadcast domains to be defined without using routers. Bridging software is used instead to define which workstations are to be included in the broadcast domain. Routers would only have to be used to communicate between two VLAN’s.Moreover , Virtual LAN. Group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.

20070725_120904_image001_207817_1285_0 16751

VLAN can do :

-Create smaller broadcast domains, and therefore less wasted bandwidth.
-Increase security, as VLANS are not visible to outside traffice
-Decrease Costs: Building with multile companies can use a single network infrastructure.
-Effecient use of bandwidth (2 trunks for a high traffic VlAN)
-Simplify management
– VLANs can also be used to help route traffice. A seperate VLAN can used for VoIP phones.
-It is also possible to seperate Wireless traffic using Wireless VLANs
– Unsecured traffic could be on a ” guest” VLAN
– Secure traffic could be on nn”Staff” VLAN

Types of Connections : 

Devices on a VLAN can be connected in three ways based on whether the connected devices are VLAN-aware or VLAN-unaware. Recall that a VLAN-aware device is one which understands VLAN memberships (i.e. which users belong to a VLAN) and VLAN formats.

1) Trunk Link: All the devices connected to a trunk link, including workstations, must be VLAN-aware. All frames on a trunk link must have a special header attached. These special frames are called tagged frames.


2) Access Link

An access link connects a VLAN-unaware device to the port of a VLAN-aware bridge. All frames on access links must be implicitly tagged (untagged).The VLAN-unaware device can be a LAN segment with VLAN-unaware workstations or it can be a number of LAN segments containing VLAN-unaware devices


3) Hybrid Link

This is a combination of the previous two links. This is a link where both VLAN-aware and VLAN-unaware devices are attached. A hybrid link can have both tagged and untagged frames, but allthe frames for a specific VLAN must be either tagged or untagged.


How to Add VLAN TO network:
Using the CL1, we enter the following on Switch: Lets it CORE1 Switch
CORE1(config) # vlan 10
CORE1(config-vlan) # name student
CORE1(config-vlan) #exit
CORE1(config) #vlan 20
CORE1(config-vlan) # name Faculty
CORE1(config-vlan) #exit
CORE1(config) #vlan 30
CORE1(config-vlan) #name struff
CORE1(config-vlan) #exit
CORE1(config) #vlan40
CORE1(config-vlan) #name guest
CORE1(config-vlan) # exit

VLANs Configuring Ports:
On each switch, identify which device is supposed to be on which VLAN. Suppose,  Student_server_core needs to be on VLAN 10. It is connected to fast ethernet interface 0/2

SWITCH(config)# int fa0/2
SWITCH(config-if)# switchport mode access
SWITCH(config-if)# switchport access vlan 20
SWITCH(config-if)# exit

* Do the same on all switches , setting the correct ports to the correct VLAN. On the device end, the only note is that all devices on a VLAN must be on the same subnet.

Trunk Link: A trunk is a point to point link between the device and another networking device. Trunk carry the traffic of multiple VLANs over single link and allow user to extend VLAN access on entire network. By default, A trunk port send traffic to add receives from all VLANS. All VLAN IDs are allowed on each trunk.

Configuration syntax for Trunk link:

Switch(config)#vlan 99

Switch(config -vlan)#exit

Switch#config t

SWITCH(config) # Interface fa0/1

Switch(config -if)# switchport mode trunk

Switch(config -if)# Switchport access trunk native vlan 99

Native VLAN: A native vlan is the untagged vlan on an 802.1q trunked switchport.  The native vlan and management vlan could be the same, but it is better security practice that they aren’t.  Basically if a switch receives untagged frames on a trunkport, they are assumed to be part of the vlan that are designated on the switchport as the native vlan.  Frames egressing a switchport on the native vlan are not tagged.

Thank you

Momataj Momo

IPv6 address Fundamental knowledge (part -1)

An IPv6 address is a 128 bit binary number and expressed in hexadecimal form, e.g.

 2001:1234:5678:0001:0000:0000:0000:0001/64 (32 hexadecimal numbers) There is a colon between each 4 hexadecimal numbers. This is for easy reading, just like the “dot-decimal form” of IPv4 address. E.g. means the first 64 bit is the network prefix, it is similar to IPv4 CIDR (Classless Inter-Domain Routing) notation 

  • Simplifying IPv6 addresses

Since it is too long to express the IPv6 address, we want to simply it.e.g. 2001:1234:5678:0001:0000:0000:0000:0001/64 can be simplified as 2001:1234:5678:1:0:0:0:1/64 

This is called “Zero compression” – The leading zeros in each segment can be omitted. Continuous zeroes can be further compressed.


  • 2001:1234:5678:1:0:0:0:1/64
  • 2001:1234:5678:1::1/64

“::” – Double Colon, means a series of 0000 groups. Since the total length of an IPv6 address is 128 bit, the number of zeroes omitted can be calculated.

 Another example:2001:0000:0000:0001:0000:0000:0000:0001/64

  • 2001:0:0:1:0:0:0:1/64
  • 2001:0:0:1::1/64
  • But Note: 2001::1::1/64 is incorrect. It is because there is no way to identify the no. of zeroes omitted in the two double-colon areas.
  • IPv6 Prefix  Let’s learn more about IPv6 Prefix. 

In IPv4, we use subnet mask to denote the network portion.

e.g. à is the network portion

It can be written as :  (CIDR notation) In IPv6, we don’t use subnet mask. We only use the latter CIDR notation e.g.


The network portion is : 2001:1234:5678:0001::  /64

The host portion is : 0000:0000:0000:0001. 

That means there can be a tremendous number of hosts, 264.

In IPv6, the network portion of an IP address is basically fixed at /64 and the host portion is always 64 bits.There is no need for subnetting. Since there are far too many bits in the IPv6 addresses that each organization can be assigned a network prefix of /48.e.g. A company may be assigned range of IP addresses with a network prefix of 2001:1234:5678:: /48. Then, the company can use 16 bits for the local subnetting.e.g. 

2001:1234:5678:0000::   /64 is the first subnet


2001:1234:5678:FFFF::   /64 is the last subnet. This results in 65536 subnets, which is far more than enough for each company or organization. In each subnet, there can be  2^64 hosts.So, the network prefix of a usable IPv6 address is basically fixed at /64 and no further subnetting is needed. This is an advantage over IPv4 because we need to do quite a lot troublesome IP address subnetting in IPv4. 

  • Demonstration

Let’s use Packet Tracer to show a demonstration of using IPv6 addresses. 




Fig: Example of Topology for IPv6

PC setting:


Router setting:


Ping test:


                                Different kinds of IPv6 addresses

  • IPv6 Global Unicast AddressI

IP addresses are allocated by IANA (Internet Assigned Numbers Authority), through 5 RIRs (Regional Internet Registries), which are responsible for 5 different areas on the Earth.


                                                          Regional Internet Registries

The current allocation of public IPv4 addresses is not sequential and continuous, meaning that a geographic region may acquire discontinuous ranges of public IPv4 address. This is due to the historical way of assignment and the insufficient public IPv4 addresses. E.g. For Macau region, it contains a large number of discontinuous, small address ranges, starting with 202.175.x, 27.x.y, 60.x.y, 113.x.y etc. This makes the aggregation of public IPv4 addresses very inefficient.

For IPv6, since it is a new deployment and there are huge numbers of IPv6 addresses. Huge enough to give each piece of sand on the Earth an IPv6 address. So, the assignment of public IPv6 addresses is more systematic. 

Currently only 1/8 of the IPv6 addresses are publicly assigned, which is :

2000::/3. What does it means? 



0010  0000  0000  0000  0000 0000 …. 0000 0000  

2      0     0    0   : 0000:0000:0000:0000:0000:0000:0000 



0011  1111  1111  1111  1111 1111 …. 1111 1111




And, currently, most of the assigned public IPv6 addresses starts with 2001::/16.

0010  0000  0000  0001  … … … … … … …… … … … … … … … … … … … … …

2     0     0     1   : … … … … … … …… … … … … … … … … … … … …



The IANA assigns address blocks to the five RIRs. The following table shows only a small portion of them. Usually, the IANA assigns address block with /23 prefix. 














                                                                Fig: IANA


So, APNIC has got this block of IPv6 addresses:


 Then, the APNIC assigns address blocks to ISPs.

e.g. APNIC may assign a block of addresses to ISPs like this : 

2001:02 55::/32  to ISPa

2001:02 66::/32  to ISPb 

So, ISPa gets a block of IP addresses as follows: 


                           fig : ISPa gets a block of IP addresses


                            fig: ISPa assigns blocks of IP addresses to different organization

In this point of view, Organization A is referred to as a Site.Now, the organization can freely use the remaining bits for its own, but, keeping 16 bits for Subnet ID. 

i.e. From 2001:0255:8888:0000::/64 to 2001:0255:8888:FFFF::/64 (The yellow portion is used as Subnet ID.)Then, for each subnet, there are 64 bits for hosts, all together, 2^64 hosts. This is called Interface ID and is used for identifying IPv6 host interface.  


                                                      fig: IPv6 host interface

 As one organization can have 65536 subnets, with each subnet having 264 hosts, this is far more than enough. So, no more subnetting is needed by the organization.

 The above resultant IPv6 addresses is publicly reachable in the Internet and is called :

1. IPv6 Global Unicast Address . It is similar to the IPv4 public addresses.


                                                   fig: IPv6 Global Unicast Address

 2. IPv6 Link local (Unicast) Address   In IPv6, a network host will try to discover if there is any neighbor nearby.e.g. PC-A may send out a message like this:


fig: IPv6 Link local (Unicast) Address (PC-A may send out a message)

And PC-B may reply:


fig: IPv6 Link local (Unicast) Address (PC -B Reply)

You will notice that they are not using their Global Unicast address. Instead, they use a kind of IPv6 address called: “Link Local address”. In IPv6, Link Local address is used to communicate with neighbors in the same link or Layer 2 segment.

How is the Link Local address formed?

link local1

                                             fig: How is the Link Local address formed?

link local11

                                                        fig: How is the Link Local address formed?

The Link Local address is automatically generated, even though the interface has not been assigned with any IPv6 Global Unicast Address. IPv6 Link Local address is analogous to IPv4 Link Local address, in the range : But, their usage is different. An IPv4 host will only get such an address when it is configured to use DHCP server to acquire IP address but no response from any DHCP server is got.


Thank you 

Momataj Momo

IPv4 : Variable Length Subnet Masking (VLSM)

A Variable Length Subnet Mask (VLSM):  is a numerical masking sequence, or IP address subset, based on overall network requirements. A VLSM allows a network administrator to use long masks for networks with few hosts and short masks for networks with multiple hosts. A VLSM is used with a VLSM router and must have routing protocol support.
A VLSM is also known as a classless Internet Protocol (IP) address.

VLSM enables you to have more than one mask for a given class of address, albeit a class A, B, or C network number.

VLSM, originally defined in RFC 1812, allows you to apply different subnet masks to the same class address space Classful protocols, such as RIPv1 and IGRP, do not support VLSM. To deploy VLSM requires a routing protocol that is classless—BGP, EIGRP, IS-IS, OSPF, or RIPv2, for instance.

VLSM provides Two major advantages:

  • more efficient use of addressing
  • Ability to perform route summarization

when you perform classful subnetting, all subnets have the same number of hosts because they all use the same subnet mask. This leads to inefficiencies. For example, if you borrow 4 bits on a Class C network, you end up with 14 valid subnets of 14 valid hosts. A serial link to another router only needs 2 hosts, but with classical subnetting, you end up wasting 12 of those hosts. Even with the ability to use NAT and private addresses, where you should never run out of addresses in a network design, you still want to ensure that the IP plan that you create is as efficient as possible.

An efficient addressing scheme using VLSM:

  • Find the largest segment in the area—the segment with the largest number of devices connected to it.
  • Find the appropriate subnet mask for the largest network segment.
  • Write down your subnet numbers to fit your subnet mask.
  • For your smaller segments, take one of these newly created subnets and apply a different, more appropriate, subnet mask to it.
  • Write down your newly subnetted subnets.
  • For even smaller segments, go back to step 4.

Variable Length Subnet Masking (VLSM) is a way of further subnetting a subnet. Using Variable Length Subnet Masking (VLSM) we can allocate IPv4 addresses to the subnets by the exact need. Variable Length Subnet Masking (VLSM) allows us to use more than one subnet mask within the same network address space. If we recollect from the previous lessons, we can divide a network only into subnets with equal number of IPv4 addresses. Variable Length Subnet Masking (VLSM) allows to create subnets from a single network with unequal number of IPv4 addresses.

Example: We want to divide, which is a Class C network, into four networks, each with unequal number of IPv4 addresses requirements as shown below.

Subnet A : 126 IPv4 Addresses.
Subnet B : 62 IPv4 Addresses.
Subnet C : 30 IPv4 Addresses.
Subnet D : 30 IPv4 Addresses.

This type of division is not possible as described in previous lessons, since it divide the network equally, but is possible with Variable Length Subnet Masking (VLSM).

Original Network (Network to be subnetted) –


Variable Length Subnet Masking (VLSM) – First Division
Divide the two networks equally with 128 IPv4 addresses (126 usable IPv4 addresses) in each network using subnet mask (

We will get two subnets each with 128 IPv4 addresses (126 usable IPv4 addresses).

1), which can be represented in binaries as below.


2), which can be represented in binaries as below.


Variable Length Subnet Masking (VLSM)- Second Division
Divide second subnet ( we got from the first division again into two Networks, each with 64 IP Addresses (62 usable IPv4 addresses) using subnet mask.

We will get two subnets each with 64 IPv4 addresses (62 usable IPv4 addresses).

1), which can be represented in binaries as below.



11000000.10101000.00001010.11 000000

Variable Length Subnet Masking (VLSM) – Third Division
Divide Network again into two Networks, each with 32 IPv4 addresses (30 usable IPv4 addresses) using subnet mask

We will get two subnets each with 32 IPv4 addresses (30 usable IPv4 addresses).

1), which can be represented in binaries as below.


2), which can be represented in binaries as below.


Now we have split the network into four subnets using Variable Length Subnet Masking (VLSM), with unequal number of IPv4 addresses as shown below. Also note that when you divide a network using Variable Length Subnet Masking (VLSM), the subnet masks are also different.

1) – (126 (128-2) usable IPv4 addresses)
2) – (62 (64-2) usable IPv4 addresses)
3) – (30 (32-2) usable IPv4 addresses)
4) – (30 (32-2) usable IPv4 addresses)

Calculation of VLSM:

The step of necessary  1. In case of VLSM , network bit borrow host bit from right side of host bits. 2. We can find out how many host exists in network . VLSM mainly divided subnet into the subnet.

Example 1: Number of user group are 500, 10 , 5 and 2. It’s needed 4 network. 



user group 500 = 29   = 512 = 9 host bits required 

First step: 172.16.0010 | 000 | 0.00000000

Network bits  |          | host bits                                  ( 000, 001,010,011,100,101)

      – 500 hosts

2nd step: 172.16.0010 | 001 | 0.00000000 /23


step 3: Next user group 10= 24   = 16

 172.16.0010 | 001 0.0000 | 0000


Step 4: for user group 5= 23   = 8

           172.16.0010 001| 0.0001 0 | 000


step 5: for user group 2= 22  = 4

     172.16.0010 001| 0.0001 1 0 | 00

Short Cut Method for VLSM:

For 2 host , need 4 bits because we need to cancel 2 usable host bits. and for find out broadcast address host bits all will be ‘1’ and we will consider highest number of user first.


Network Address

Broadcast Address

500 host = 29

               = 512


10 host =24

              =16 /28

5 host =23

              = 8

2 host = 22 = 4                                  

 Example 2: , user group are 1000, 1000, 50 , 100, 2, 2


Network Address

Broadcast Address

1000 host = 210

               = 1024


1000 host = 210

               = 1024 /22

100 host = 27

               = 128

50 host = 26

               = 64 (64-1=63 host) (128+63=191 host)

2 host = 22

               = 4 (n-1) bit added

CIDR (32-2 =30) /30 (192+3 =195)

2 host = 22

               = 4 (n-1) bit added



Thank you

Momataj Momo


Internet protocol (IPV4) Version 4

Internet Protocol version 4 is the fourth version in the development of the Internet Protocol Internet, and routes most traffic on the Internet.

IPv4 uses 32-bit (four-byte) addresses, which limits the address space to 4294967296 (232) addresses.As addresses were assigned to users, the number of unassigned addresses decreased.

It had been significantly delayed by address changes such as classful network design, Classless Inter-Domain Routing, and network address translation (NAT).

IPv4 reserves special address blocks for private networks (~18 million addresses) and multicast addresses (~270 million addresses).

IANA coordinates allocations from the global IP and AS number spaces, such as those made to Regional Internet Registries.It is US based Organization. Its control IP,MAC or control any others number of network.

  • IP Addresses & AS Numbers
  • Network abuse information

Address representations: 
IPv4 addresses may be written in any notation expressing a 32-bit integer value, but for human convenience, they are most often written in the dot-decimal notation, which consists of four octets of the address expressed individually in decimal and separated by periods.


Fig: IPV4 address Assign


Fig; Divided 32- bits IPV4 per segment 8 bits


An IP address was divided into two parts: Network Address and Host Address 

the network identifier was the most significant (highest order) octet of the address, and the host identifier was the rest of the address. The latter was therefore also called the rest field. This enabled the creation of a maximum of 256 networks.

Allocaton of ip

fig: Allocation of IPv4 address

The system defined five classes, Class A, B, C, D, and E. The Classes A, B, and C had different bit lengths for the new network identification. The rest of an address was used as previously to identify a host within a network, which meant that each network class had a different capacity to address hosts. Class D was allocated for multicast addressing and Class E was reserved for future applications.

IPV4 Address Ranges : 

Class A  ———————–          to 

Class B ————————      to 

Class C————————-      to  

Class D ————————      to   using for multitasking 

Class E ————————     to     reserve for future research 


Starting around 1985, methods were devised to subdivide IP networks. One method that has proved flexible is the use of the variable-length subnet mask (VLSM).

Based on the IETF standard RFC 1517 published in 1993, this system of classes was officially replaced with Classless Inter-Domain Routing (CIDR), and the class-based scheme was dubbed classful, by contrast. CIDR was designed to permit repapartitioning any address space so that smaller or larger blocks of addresses could be allocated to users.

The hierarchical structure created by CIDR is managed by the Internet Assigned Numbers Authority (IANA) and the regional Internet registries (RIRs). Each RIR maintains a publicly searchable WHOIS database that provides information about IP address assignments.

Types of IP Address :

IP address has divided into two parts: 

a) Public IP address : public address using in Gateway / Router for using internet,

b) Private IP address : Private IP address has using into LAN connection.

Private IP and networks: 

Of the approximately four billion addresses allowed in IPv4, three ranges of address are reserved for use in private networks. These ranges are not routable outside of private networks, and private machines cannot directly communicate with public networks. Private IP limit using in LAN inside . 

 Private IP range :




private ip

Virtual private networks:

Packets with a private destination address are ignored by all public routers. Two private networks (e.g., two branch offices) cannot communicate via the public internet, unless they use an IP tunnel or a virtual private network (VPN). When one private network wants to send a packet to another private network, the first private network encapsulates the packet in a protocol layer so that the packet can travel through the public network. Then the packet travels through the public network. When the packet reaches the other private network, its protocol layer is removed, and the packet travels to its destination.

 Optionally, encapsulated packets may be encrypted to secure the data while it travels over the public network.

Link-local address :

RFC 6890 defines the special address block for link-local addressing. hese addresses are only valid on links (such as a local network segment or point-to-point connection) connected to a host. These addresses are not routable. Like private addresses, these addresses cannot be the source or destination of packets traversing the internet.

When the address block was reserved, no standards existed for address autoconfiguration. Microsoft created an implementation called Automatic Private IP Addressing (APIPA), which was deployed on millions of machines and became a de facto standard.

LoopBack / local host address:

The class A network (classless network is reserved for loopback. IP packets whose source addresses belong to this network should never appear outside a host. The modus operandi of this network expands upon that of a loopback interface:

  • IP packets whose source and destination addresses belong to the network (or subnetwork) of the same loopback interface are returned to that interface;
  • IP packets whose source and destination addresses belong to networks (or subnetworks) of different interfaces of the same host, one of them being a loopback interface, are forwarded regularly.

Address Ending in 0 or 255:

Networks with subnet masks of at least 24 bits, i.e. Class C networks in classful networking, and networks with CIDR suffixes /24 to /32 (– may not have an address ending in 0 or 255.

Classful addressing prescribed only three possible subnet masks: Class A, or /8; Class B, or /16; and Class C, or /24. For example, in the subnet ( the identifier commonly is used to refer to the entire subnet. To avoid ambiguity in representation, the address ending in the octet 0 is reserved.

A broadcast address is an address that allows information to be sent to all interfaces in a given subnet, rather than a specific machine. Generally, the broadcast address is found by obtaining the bit complement of the subnet mask and performing a bitwise OR operation with the network identifier. In other words, the broadcast address is the last address in the address range of the subnet. For example, the broadcast address for the network is For networks of size /24 or larger, the broadcast address always ends in 255.

note: In networks smaller than /24, broadcast addresses do not necessarily end with 255. For example, a CIDR subnet has the broadcast address

Address Resolution : Domain Name system

Hosts on the Internet are usually known by names, e.g.,, not primarily by their IP address, which is used for routing and network interface identification. The use of domain names requires translating, called resolving, them to addresses and vice versa.

The translation between addresses and domain names is performed by the Domain Name System (DNS), a hierarchical, distributed naming system which allows for subdelegation of name spaces to other DNS servers.

Packet Structure :

An IP packet consists of a header section and a data section.An IP packet has no data checksum or any other footer after the data section. Typically the link layer encapsulates IP packets in frames with a CRC footer that detects most errors, and typically the end-to-end TCP layer checksum detects most other errors.

Header :

The IPv4 packet header consists of 14 fields, of which 13 are required. The 14th field is optional (red background in table) and aptly named: options. The fields in the header are packed with the most significant byte first (big endian), and for the diagram and discussion, the most significant bits are considered to come first (MSB 0 bit numbering). The most significant bit is numbered 0, so the version field is actually found in the four most significant bits of the first byte, for example.

Version The first header field in an IP packet is the four-bit version field. For IPv4, this has a value of 4 (hence the name IPv4).

Related Topics :

  • Intranet is shared content accessed by members within a single organization.(An intranet is a private computer network that uses Internet Protocol technologies to securely share any part of an organization’s information or operational systems within that organization)
  • Extranet is shared content accessed by groups through cross-enterprise boundaries.(An extranet is a private network that uses Internet protocols, network connectivity. An extranet can be viewed as part of a company’s intranet that is extended to users outside the company, usually via the Internet.)
  • Internet is global communication accessed through the Web.(The Internet is a global system of interconnected computer networks that use the standard Internet Protocol Suite (TCP/IP) to serve billions of users worldwide.)
  • A local area network (LAN) is a computer network that interconnects computers within a limited area such as a home, school, computer laboratory, or office building, using network media. Its need Mac address of every connected computers for communication and Its used Switch as Intermediary Device. For LAN connection IP address is not necessary .LAN should have to same network address. 
  • A router is a networking device, commonly specialized hardware, that forwards data packets between computer networks. This creates an overlay internetwork, as a router is connected to two or more data lines from different networks. When a data packet comes in one of the lines, the router reads the address information in the packet to determine its ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey. Routers perform the “traffic directing” functions on the Internet. Router is called Gateway when need to connect Internet.

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Momataj Momo

Description of Network Layer and related layers

In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.It is work with IP protocol and default device is router.


The network layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions.

Functions of the network layer include:

  • Connection model: connectionless communication
    For example, IP is connectionless, in that a datagram can travel from a sender to a recipient without the recipient having to send an acknowledgement. Connection-oriented protocols exist at other, higher layers of the OSI model.
  • Host addressing: Every host in the network must have a unique address that determines where it is. This address is normally assigned from a hierarchical system.. On the Internet, addresses are known as Internet Protocol (IP) addresses.
  • Message Forwarding : Since many networks are partitioned into subnetworks and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers, to forward packets between networks. This is also of interest to mobile applications, where a user may move from one location to another, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was not designed with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.

OSI network architecture, the network layer responds to service requests from the transport layer and issues service requests to the data link layer.

Transport layer to physical layer data forwarding system: 


Fig: Data sent in transport layer

network layer

Fig: Packet forwarding in network layer


Fig: Frame send in Data link layer


Fig: Frame become single after come physical layer

A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents; on retrieval the calculation is repeated, and corrective action can be taken against presumed data corruption if the check values do not match.

CRCs are so called because the check (data verification) value is a redundancy (it expands the message without adding information) and the algorithm is based on cyclic codes. 

Two kind of Data packet sent in network layer:

a) User Datagram Protocol (UDP): UPDThe User Datagram Protocol (UDP) is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol (IP) network without requiring prior communications to set up special transmission channels or data paths.

Data Structure of UDP:

  • UDP is a minimal message-oriented Transport Layer protocol that is documented in IETF RFC 768.
  • UDP provides no guarantees to the upper layer protocol for message delivery and the UDP protocol layer retains no state of UDP messages once sent. For this reason, UDP sometimes is referred to as Unreliable Datagram Protocol.
  • UDP provides application multiplexing (via port numbers) and integrity verification (via checksum) of the header and payload. If transmission reliability is desired, it must be implemented in the user’s application.
  • The UDP header consists of 4 fields, each of which is 2 bytes (16 bits).The use of the fields “Checksum” and “Source port” is optional in IPv4 (pink background in table). In IPv6 only the source port is optional.
  • When UDP runs over IPv4, the checksum is computed using a “pseudo header” that contains some of the same information from the real IPv4 header. The pseudo header is not the real IPv4 header used to send an IP packet, it is used only for the checksum calculation.
  • When UDP runs over IPv6, the checksum is mandatory. The method used to compute it is changed as documented in RFC 2460

Most often, UDP applications do not employ reliability mechanisms and may even be hindered by them. Streaming media, real-time multiplayer games and voice over IP (VoIP) are examples of applications that often use UDP. In these particular applications, loss of packets is not usually a fatal problem. If an application requires a high degree of reliability, a protocol such as the Transmission Control Protocol may be used instead.

b) Protocol Data unit (PDU):PDU Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data.

PDUs are relevant in relation to each of the first 4 layers of the OSI model as follows:

The Layer 1 (Physical Layer) PDU is the packet, consisting of bits or, more generally, symbols (can also be seen as “stream”)
The Layer 2 (Data Link Layer) PDU is the frame
The Layer 3 (Network Layer) PDU is the packet
The Layer 4 (Transport Layer) PDU is the segment for TCP, or the datagram for UDP
The Layer 5-6-7 (Application Layer) PDU is the message
Given a context pertaining to a specific OSI layer, PDU is sometimes used as a synonym for its representation at that layer.

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Momataj Momo


Introduction to Network Protocol and OSI model

Protocol is the set of rules and regulation.

A network protocol defines rules and conventions for communication between network devices. Protocols for computer networking all generally use packet switching techniques to send and receive messages in the form of packets.

The most common network protocols in public use belong to the Internet Protocol (IP) . IP is itself the basic protocol that enables home and other local networks across the Internet to communicate with each other.

The Transmission Control Protocol (TCP) protocol extends IP with this higher layer capability, and because point-to-point connections are so essential on the Internet, the two protocols are almost always paired together and known as TCP/IP. Both TCP and IP operate somewhere in the middle layers of a network protocol stack. TCP/IP in turn runs on top of lower-level network technologies like Ethernet

Different Types of Protocols : 

HyperText Transfer Protocol (HTTP) is used by Web browsers and servers worldwide. port number is 80.

Other popular network protocols in the IP family include ARP, ICMP,POP, SMTP, MUA and FTP

File Transfer Protocol (FTP) allows you to transfer files between two computers on the Internet.FTP is a simple network protocol based on iP and also a term used when referring to the process of copying files when using FTP technology.

Default Port Number for FTP is 21.

Post Office Protocol (POP): The Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP has been developed through several versions, with version 3 (POP3) being the current standard.

Virtually all modern e-mail clients and servers support POP3, and it along with IMAP (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval,with many webmail service providers such as Gmail, and Yahoo! Mail also providing support for either IMAP or POP3 to allow mail to be downloaded.

Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission. It also known as Short message transfer protocol.

SMTP by default uses TCP port 25. The protocol for mail submission is the same, but uses port 587. SMTP connections secured by SSL, known as SMTPS, default to port 465.

While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For receiving messages, client applications usually use either POP3 or IMAP.

Mail user agent (MUA) is a computer program used to access and manage a user’s email.Its known as also An email client, email reader. Popular web-based email clients include: Gmail, Lycos Mail,, and Yahoo! Mail.

The Open Systems Interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.

OSI model




Fig : TCP/IP Layer


350px-IP_stack_connections.svg (1)

Fig: Data Flow system


Major Function of OSI and TCP/IP Layer :

1. Physical Layer:

It defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable).It also defines the protocol to establish and terminate a connection between two directly connected nodes over a communications medium.

– Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.

2. Data Link Layer:

The data link layer provides a reliable link between two directly connected nodes, by detecting and possibly correcting errors that may occur in the physical layer. The data link layer is divided into two sublayers:

  • Media Access Control (MAC) layer – responsible for controlling how computers in the network gain access to data and permission to transmit it.
  • Logical Link Control (LLC) layer – control error checking and packet synchronizatio.

The Point-to-Point protocol (PPP) is an example of a data link layer in the TCP/IP protocol stack.Switch Device works in this layer .

– Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay, 


3. Network Layer : 

The network layer provides the functional and procedural means of transferring variable length data sequences (called datagrams) from one node to another connected to the same network.Datagram delivery at the network layer is not guaranteed to be reliable.This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.Router Device works on this layer.

Layer 3 Network examples include AppleTalk DDP, IP, IPX.

4. Transport Layer: 

The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions.This layer provides transparent transfer of data between end systems, or hosts and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

– Layer 4 Transport examples include SPX, TCP, UDP.

5. Session Layer:

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.

– Layer 5 Session examples include NFS, NetBios names, RPC, SQL.

6. Presentation Layer:

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

– Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

7. Application Layer:

The application layer is the OSI layer closest to the end user, which means both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.

This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

– Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP


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Momataj Momo