Subneting - Subnets

The following table is probably the most useful table to remember when taking any Cisco exam, or just wanting to work out the subnet mast you will need to get a certain number of hosts and networks.

Mask	Slash	Subnet Bits	Networks (2 to the power of Subnet bits)*	Host Bits	Hosts (2 to the power of Host bits ? 2)
.0	/24	0	1	8	254
.128	/25	1	2	7	126
.192	/26	2	4	6	62
.224	/27	3	8	5	30
.240	/28	4	16	4	14
.248	/29	5	32	3	6
.252	/30	6	64	2	2

* note: the Networks column is calculated assuming ?ip zubnet-zero? is switched ON. Subnet Zero means that you can have ?all-zeros? subnets. Under old IP subnetting rules, the all 0?s subnet was reserved for the network, and the all 1?s subnet was reserved for the broadcast. Over time, engineers found that the all 0?s subnet wasn?t really used and, if it could be handed out as a useable network, many IP addresses could be changed.

An example of an IP address that is using a zero subnet is 10.1.0.1 with a subnet mask of 255.255.255.0. This IP address may look strange. Some people may even argue that it is an invalid IP address because there is a 0 in third octet. However, today, this IP address is perfectly legal when it comes to subnetting. Thus, if you had an IP address of 10.1.0.0 with a 255.255.0.0 subnet mask and wanted to subnet it, I could actually get 255 valid networks out of it by using the 0 subnet. In other words, I could have networks ranging from 10.1.{0-254}.X where the X represents hosts 1-254. This gives me room for networks 0-254, or 255 total networks, by using the 0 subnet.