Ques- What is the significance of the following TCP header fields:

  • Urgent Pointer
  • Checksum
  • Window Size
  • Reserved bits
  • Sequence Number
Ans- In the Transmission Control Protocol (TCP) Segment Header lesson, you will learn more about TCP Segment Header, different fields in TCP Header and the use of these fields.

Transmission Control Protocol (TCP) Segment Header.

Sequence number

32 Bit number used for byte level numbering of TCP segments. If you are using TCP, each byte of data is assigned a sequence number. If SYN flag is set (during the initial three way handshake connection initiation), then this is the initial sequence number. The sequence number of the actual first data byte will then be this sequence number plus 1. For example, let the first byte of data by a device in a particular TCP header will have its sequence number in this field 50000. If this packet has 500 bytes of data in it, then the next packet sent by this device will have the sequence number of 50000 500 1 = 50501.

Reserved Bits

  • These bits are not used.
  • The 6 bits are reserved.

Window size

Indicates the size of the receive window, which specifies the number of bytes beyond the sequence number in the acknowledgment field that the receiver is currently willing to receive.

  • Thus, window size is used for Flow Control.
  • It advertises how much data (in bytes) the sender can receive without acknowledgement.
  • It contains the size of the receiving window of the sender.
  • Window size is a 16 bit field.


  • Receiver rejects the data that fails the CRC check.
  • Sender adds CRC checksum to the checksum field before sending the data.
  • It verifies the integrity of data in the TCP payload.
  • Checksum is a 16 bit field used for error control.

Urgent Pointer

Shows the end of the urgent data so that interrupted data streams can continue. When the URG bit is set, the data is given priority over other data streams (Size 16 bits).


Subnetting is the practice of dividing a network into two or more smaller networks. It increases routing efficiency, enhances the security of the network and reduces the size of the broadcast domain.

Consider the following example:

In the picture above we have one huge network: All hosts on the network are in the same subnet, which has the following disadvantages:

organizational problems – in a large networks, different departments are usually grouped into different subnets. For example, you can group all devices from the Accounting department in the same subnet and then give access to sensitive financial data only to hosts from that subnet.

network security – each device can reach any other device on the network, which can present security problems. For example, a server containing sensitive information shouldn’t be in the same network as user’s workstations.

a single broadcast domain – all hosts are in the same broadcast domain. A broadcast sent by any device on the network will be processed by all hosts, creating lots of unnecessary traffic.

Subnet mask

An IP address is divided into two parts: network and host parts. For example, an IP class A address consists of 8 bits identifying the network and 24 bits identifying the host. This is because the default subnet mask for a class A IP address is 8 bits long. (or, written in dotted decimal notation, What does it mean? Well, like an IP address, a subnet mask also consists of 32 bits. Computers use it to determine the network part and the host part of an address. The 1s in the subnet mask represent a network part, the 0s a host part.

Computers works only with bits. The math used to determine a network range is binary AND.

binary and

Let’s say that we have the IP address of with the default subnet mask of 8 bits (
First, we need to convert the IP address to binary:

IP address: = 00001010.00000000.00000000.00000001
Subnet mask = 11111111.00000000.00000000.0000000

Computers then use the AND operation to determine the network number:

determining the network number

The computer can then determine the size of the network. Only IP addresses that begins with 10 will be in the same network. So, in this case, the range of addresses in this network is –

A subnet mask must always be a series of 1s followed by a series of 0s.

DEFAULT subnet mask

By now you should have some idea what the subnet mask does and how it's used to partition a network. What you need to keep in mind is that each Class has its DEFAULT subnet mask, which we can change to suit our needs. I have already mentioned this in the previous page, but we need to look into it in a bit more detail.

 IP address is an address having information about how to reach a specific host, especially outside the LAN. An IP address is a 32 bit unique address having an address space of 232. Generally, there are two notations in which IP address is written, dotted decimal notation and hexadecimal notation.

Classful Addressing

In Classful addressing, the address space is divided into five classes: A, B, C, D, and E. Each of these classes has a valid range of IP addresses. Classes D and E are reserved for multicast and experimental purposes respectively. The order of bits in the first octet determine the classes of IP address.

IPv4 address is divided into two parts:


  • Network ID
  • Host ID


The class of IP address is used to determine the bits used for network ID and host ID and the number of total networks and hosts possible in that particular class. Each ISP or network administrator assigns an IP address to each device that is connected to its network.



Note: While finding the total number of host IP addresses, 2 IP addresses are not counted and are, therefore, decreased from the total count because the first IP address of any network is the network number and whereas the last IP address is reserved for broadcast IP.

The Classful addressing wastes a large part of the address space.

  • Class A: 0----2^7---- 2^24
  • Class B: 10--- 2^14--- 2^16
  • Class C: 110---- 2^21--- 2^8
  • Class D: 1110---1------ 2^28


Problems with Classful Addressing: 

The problem with this classful addressing method is that millions of class A address are wasted, many of the class B address are wasted, whereas, number of addresses available in class C is so small that it cannot cater the needs of organizations. Class D addresses are used for multicast routing and are therefore available as a single block only. Class E addresses are reserved. 
Since there are these problems, Classful networking was replaced by Classless Inter-Domain Routing (CIDR) in 1993.