Internet IP Subnetting

Internet IP Subnetting

IP Subnetting is a method of segmenting hosts within a network and providing additional structure. Without subnets, an organization operates as a flat network. These flat topologies result in short routing tables, but as the network grows, the use of bandwidth becomes inefficient.

Routers use a subnet mask to determine which parts of the IP address correspond to the network, the subnet, and the host. The mask is a 32-bit number in the same format as the IP address. The mask is a string of consecutive is starting from the most significant bits representing the network ID, followed by a string of consecutive Os representing the host ID.

IP Address	Network		Host
	128 10000000	10 00001010	173 10110010	46 00101110

Subnet Mask	Network		Subnet	Host
	255 11111111	255 11111111	255 11111111	0 00000000

This subnet mask can also be written as "/24", where 24 represents the number of 1s in the subnet mask.

Each address class has a default subnet mask (A=/8, B=/16, C=124). The default subnet masks only the network portion of the address; the effect is no subnetting. With each bit of subnetting beyond the default, you can create 2-2 subnets. The preceding example has 254 subnets, each with 254 hosts.

Identifying Subnet Addresses
Given an IP address and subnet mask, you can identify the subnet address, broadcast address, and the first and the last usable addresses within a sub- net as follows:

• Write the 32-bit address and subnet mask below that (the following figure shows 174.24.4.176/26).
• Draw a vertical line just after the last 1 bit in the subnet mask.
• Copy the portion of the IP address to the left of the line. Place all Os for the remaining free spaces to the right. This is the subnet number.
• Copy the portion of the IP address to the left of the line. Place all is for the remaining free spaces to the right. This is the broadcast address for the subnet.
• You can also find the first and last address by placing ...000i and ...iliO respectively in the remaining free spaces.

174.24.4.176	1010111000110000000100	10|110000	Host
255.255.255.192	1111111111111111111111	11|000000	Mask
174.24.4.128	1010111000110000000100	10|000000	Subnet
174.24.4.191	1010111000110000000100	10|111111	Broadcast