What does the last part of an internet router's IP address mean?
We have a router that has a public IP address in the form of #.#.#.#/28. What does the '/28' mean? Could it mean that I have 28 total public IP addresses? Example: If the IP address was 24.48.96.100/28, then would this mean I have public addresses in the range of 24.48.96.100 through 24.48.96.27?
Public Comments
1. The /28 is a notation for the subnet mask. The subnet mask is used to differentiate the network portion of your IP address from the client portion of the IP address. So, in a way, you are correct about the public/private thing, but it doesn't work the way you think it does. (A /28 subnet mask is also commonly indicated in the form 255.255.255.240).The /28 tells you the number of contiguous bits that make up the subnet mask. As you know, there are four octets in an IP address (each seperated by the ' . '). Each of those octets is coded by eight bits. So a subnet mask of /28 tells you your public subnet uses all of the first octet (8 bits), all of the second octet (16 bits), all of the third octet (24 bits) and the first four of the remaining eight bits in the last octet. (There are 32 bits in total).
The way to calculate the number of private addresses you get from a particular subnet mask is to take the number of bits remaining and apply this formula:
2 ^ (n-2) [n = the number of internal bits]
So if you have a subnet mask of /28, you have 4 bits left for your private network. Plug that into the formula and you get:
2 ^ (4-2)
2 ^ (2)
4
So with a subnet mask of /28, you actually have 4 available public IP addresses.
Working through IP schemes can have a bit of a learning curve. What I went into here was just glossing the surface. If you want to learn more, do a search for IP addressing... you'll find tons of literature on the net.
