Chapter 12

SubnettingCIS 1140

Subnetting Subnetting: taking a single network address

issued from a registrar or an ISP and creating two or more subnetworks A subnet is a network created by borrowing bits

from the host portion of an assigned network IP address.

Binary number system is used for IP addressing.

Even though binary number system is used for addressing, IP addresses are typically written in dotted decimal format. Ex. 172.16.15.3

Subnetting (cont.) 1. Determine the class of the IP address to

be used and all accompanying information: 0 – 127 Class A

N – H – H – H 255.0.0.0

128 – 191 Class B N – N – H – H 255.255.0.0

192 – 223 Class C N – N – N – H 255.255.255.0

Subnetting (cont.) 2. Determine how many bits to “borrow” for

subnetting based on the number of networks or hosts that must be accommodated You must leave at least 2 bits for hosts. 2n - 2 ≥ number of subnets needed (where n is

the number of bits borrowed for subnetting) 2n - 2 ≥ number of hosts needed (where n is the

number of bits left for addressing hosts)

Subnetting (cont.) How many bits would need to be borrowed

to produce 7 subnets?2n - 2 ≥ 72n ≥ 9n = 4 (remember 24=16)

How many bits would need to be borrowed to produce 25 subnets?2n - 2 ≥ 252n ≥ 27n = 5 (remember 25=32)

Subnetting (cont.) How many bits would need to be borrowed

to produce 220 subnets?2n - 2 ≥ 2202n ≥ 222n = 8 (remember 28=256)

How many bits would need to be used to produce 12 hosts per subnet?2n - 2 ≥ 122n ≥ 14n = 4 (remember 24=16)

Subnetting (cont.) How many bits would need to be used to

produce 35 hosts per subnet?2n - 2 ≥ 352n ≥ 37n = 6 (remember 26=64)

How many bits would need to be used to produce 500 hosts per subnet?2n - 2 ≥ 5002n ≥ 502n = 9 (remember 29=512)

Subnetting (cont.) How many bits are borrowed for subnetting

a class B address if 10 must be used for hosts?8 bits from the 3rd octet + 8 bits from the 4th octet give

16 bits for hosts + subnets16 – 10 = 6

How many bits are borrowed for subnetting a class C address if 3 must be used for hosts?

8 bits from the 4th octet give 8 bits for hosts + subnets

8 – 3 = 5

Subnetting (cont.)

3. Determine the actual subnet mask of the newly created networks Start with the default subnet mask and

add in the bits that were “borrowed” or set during the subnetting process If three bits were borrowed from a class C

address:128 64 32 16 8 4 2 1 1 1 1 0 0 0 0 0 = 224 (128+64+32)

The mask would be 255.255.255.224

Subnetting (cont.)

If four bits are borrowed from a class C address:

128 64 32 16 8 4 2 1 1 1 1 1 0 0 0 0 = 240 (128+64+32+16) The mask would be 255.255.255.240

If four bits are borrowed from a class B address:

128 64 32 16 8 4 2 1 1 1 1 1 0 0 0 0 = 240 (128+64+32+16) The mask would be 255.255.240.0

Subnetting (cont.) 4. Determine what value you are ranging on. The last

bit turned on in an octet will determine the range. Borrow 3 bits for subnetting 192.168.3.0

128 64 32 16 8 4 2 1 1 1 1 0 0 0 0 0 This would range on 32.192.168.3.0 192.168.3.128192.168.3.32 192.168.3.160192.168.3.64 192.168.3.192192.168.3.96 192.168.3.224*Note the equation 2n - 2 ≥ subnets When n = 3 it is

predicted that 6 useable subnets will be produced. When the first and last subnet are thrown out this is true. This would leave 5 bits for hosts. 2n - 2 ≥ hosts would predict 30 useable hosts which is also true.

Subnetting (cont.) The first address in a network or subnet is used to identify

the subnet/network and cannot be used to address hosts. The last address in a network or subnet is used as the

broadcast for the subnet/network and cannot be used to address hosts.

When a subnet is created, we typically throw out the first and last subnet to avoid confusion: In the previous example:

Does 192.168.3.0 identify the entire 192.168.3.0 network or just the first subnet?

Does 192.168.3.255 create a broadcast for the entire 192.168.3.0 network or just the subnet 192.168.3.224?

This throwing out of two subnets and two addresses in each subnet explains the – 2 in the equation 2n - 2 ≥ number of hosts/subnets

Subnetting (cont.) Borrow 4 bits for subnetting 192.168.3.0

128 64 32 16 8 4 2 1 1 1 1 1 0 0 0 0 This would range on 16.192.168.3.0 192.168.3.128192.168.3.16 192.168.3.144192.168.3.32 192.168.3.160192.168.3.48 192.168.3.176192.168.3.64 192.168.3.192192.168.3.80 192.168.3.208192.168.3.96 192.168.3.224192.168.3.112 192.168.3.240*Note the equation 2n - 2 ≥ subnets When n = 4 it is

predicted that 14 useable subnets will be produced. When the first and last subnet are thrown out this is true. This would leave 4 bits for hosts. 2n - 2 ≥ hosts would predict 14 useable hosts which is also true.

Variable Length Subnet Mask Variable Length Subnet Mask (VLSM): a subnet mask

that is not expressed in standard 8-bit or one-byte values. A VLSM occurs when a subnet is further divided into smaller subnets, which are not equal to the original subnet in length or in the number of hosts. Subnetting a subnet Avoids wasting addresses when subnets vary greatly

in size Ex. Point-to-point links really require only 4 address: 1

ID 1 Broadcast 2 one address for each side of the link Fixed Length subnet Mask (FLSM): subnets are equal

in range and have an equal number of hosts

Advantages of Subnetting Creates a more secure network by

placing hosts on separate networks. Breaks up broadcast domains. Broadcast

traffic from one network does not “bleed” over into other networks.

Fewer collision occur because of reduced broadcasts and segmentation reduces the size of the collision domains.

Disadvantages of Subnetting Subnetting can become confusing as

networks grow larger. Subnetting itself is not an easy

process if the administrator does not practice.

Not all device and protocols can use all types of subnetting. Ex. Not all routing protocols support

VLSM.