What does my ISP mean when they say they will provide "4 static IPs out of which 2 are usable"? [duplicate]

I am planning to get leased line installed at my office. I have talked to one of the ISPs about static IP. They said I'll get 4 static IPs out of which 2 are usable. What does that mean?

They are saying that one is the broadcast, one is the net base, and 2 are usable by the customer. Sounds like you are getting a /30 block.

Just to get a better understanding, check out the authoritative thread on IP subnetting.

How does IPv4 Subnetting Work?

This means you will get a /30 network for you to use.

If you have a network (lets say 192.168.0.0/30), possible addresses are:

192.168.0.0 <- this is your network ID, unusable for 'normal' traffic
192.168.0.1 <- 1st usable IP
192.168.0.2 <- 2nd usable IP
192.168.0.3 <- broadcast address, unusable for 'normal' traffic

So you basically get 4 addresses, out of which, only 2 are usable for PCs/servers/routers/....

The other answers already covered it, but I wanted to add two more points:

First, the Mulaz' answer with binary also shown:

192.168.0.0/30
11000000.10101000.00000000.00000000/30
NNNNNNNN NNNNNNNN NNNNNNNN NNNNNNHH 

30 N's for the network
02 H's for usuable IPs

Written in binary:                   |  In dotted decimal  | description
---------------------------------------------------------------------------------------------------
11000000 10101000 00000000 00000000  |  192.168.0.0        | technically usable
11000000 10101000 00000000 00000001  |  192.168.0.1        | 1st *safely* usable IP
11000000 10101000 00000000 00000010  |  192.168.0.2        | 2nd *safely* usable IP
11000000 10101000 00000000 00000011  |  192.168.0.3        | broadcast address, unusable for 'normal' traffic

Secondly, there is a good chance that you will not get a block ending on .0,
instead you might get something like this:

192.168.0.4/30
192.168.0.00000100/30

192.168.0.4  this is your network ID
192.168.0.5  1st usable IP
192.168.0.6  2nd usable IP
192.168.0.7  broadcast address

Lastly, iff you have full control of each part of the network then you can also use the network address as a usuable IP. And I stress iff (if and only if). Many devices still use or recognise the first address as an alternative broadcast address. If you have full control over every piece of hardware and over the OS then you can use it. If you ever consider connecting it to an other network (e.g. your ISP) then you will have problems.