/30 BIT IP Subnet Mask
Subnet masks that look like 255.255.255.0. This is a /24 subnet mask in "slash notation". It is much easier to type /24 than it is to type 255.255.255.0. These two are the same because if you translate 255.255.255.0 to binary, you get 11111111 11111111 11111111 00000000, or 24 one's.
A /24 bit subnet mask has 254 usable IP addresses + 1 for the broadcast + 1 for the network. This is calculated 2^8 (or 2 to the 8th power) = 256 - 2 = 254.
A /30 bit mask would be 30 one's, leaving just 2 zero's that could be used for host addressing. If you apply the hosts formula, you get 2^2 = 4 - 2 = 2 useable IP addresses. In other words, our network would look like this:
- Network
- Host IP
- Host IP
- Broadcast
How can /30 bit subnet masks help in in IP addressing the WAN?
The point of proper IP address design is that you don't use any more IP addresses than are absolutely needed. With a /30 bit subnet mask providing us 2 useable IP addresses, Think of a network where there are only 2 hosts. While this idea seems silly for IP Addressing on a LAN, it is actually perfect for IP addressing on WAN connections or any router to router connections.
For example, take an Internet T1 circuit. You setup the router and connect to the ISP's router. An Internet circuit is treated like a point to point WAN circuit. There are only 2 routers on that circuit, each connecting to each other. Then you need a network address so that that entire network can be represented in a routing table.
A /30 subnet mask that only provides 2 useable IP addresses put to work on a network with only 2 devices - a point to point T1 Internet circuit. Also, this example completely applies to a point to point private network leased line (not on the Internet).
Again, the /30 subnet mask provides the most efficient use of IP addresses by not wasting any IP addresses when it is applies to a point to point network connection.
With a Cisco router point to point connection, a /3'02 bit network in use would look like:
Router 1
Serial 0/0
ip address 1.1.1.1 255.255.255.252
Router 2
Serial 0/0
ip address 1.1.1.2 255.255.255.252IT Terms
