CS 447 Networks and Data Communication

IP Subnet Mask:

Purpose: To indicate which portion of an IP address indicates host address

Example

SIUE IP Network Domain: 146.163.XX.XX

10010000 10100011 XXXXXXXX XXXXXXXX

146 163 XX XX

If we do NOT have any subnetwork

We know that the last 16 bits indicate the host addresses(since a Class-B IP address assumes to use the last 16 bits for host address)

Subnet_Mask/001

CS 447 Networks and Data Communication

Owners of a network domain can create as many subnets as they want.

Problem

1 bit to indicate subnetwork

2 bits to indicate subnetwork

If there are two subnetworks within a domain

If there are four subnetworks within a domain

If there are 2n subnetworks within a domain n bits to indicate subnetwork

10010000 10100011 XXXXXXXX XXXXXXXX

146 163 XX XX

We need to indicate how many bits are assigned for subnetworksSubnet_Mask/002

CS 447 Networks and Data Communication

10010000 10100011 01101101 00000001146.163.109.1:

- If SIUE has 16 subnetworks- Try to figure out the host address of 146.163.109.1

How to use Subnet Mask #1: Get Host Address

BINARY AND

13 1

00000000 00000000 00001111 11111111Invert 1 and 0s

00000000 00000000 00001101 00000001

11111111 11111111 11110000 00000000

Mask for domain address

Mask for subnetwork

Subnet Mask:

Subnet_Mask/003

(255.255.240.0)

CS 447 Networks and Data Communication

- If SIUE has 16 subnetworks- Try to figure out the domain and subnetwork address of 146.163.109.1

How to use Subnet Mask #2: Get Domain Address and Subnet Address

Mask for domain address

10010000 10100011 01101101 00000001146.163.109.1:

BINARY AND

10010000 10100011 01100000 00000000

11111111 11111111 11110000 00000000Subnet Mask:

IP domain address Subnetwork address

Subnet_Mask/004

Subnet Indicator

CS 447 Networks and Data Communication

Why do we need “Subnet Mask”

Subnet_Mask/005

• The IP defines three classes of IP: Class-A, Class-B and Class-C

• Each class defines the number of hosts within a network, but does not define the number of sub-nets within a network.

• As long as your network does not have any sub-network, you do not have to use “Subnet Mask”

• Since each network can have different number of sub-networks, we have to let routers know which bits are used for identifying subnets