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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?

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marked as duplicate by Chris S Feb 27 '13 at 23:37

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4 Answers 4

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?

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Thanks for the clarification and for the swift reply.. –  user162140 Feb 27 '13 at 18:22

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/....

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Thanks.. I had a doubt and now its clear!!! :) –  user162140 Feb 27 '13 at 18:20
    
Ah yes, the /30 netblock. This is usually set up as a routing block between an ISP and an existing network. It's not really that useful as an end-client subnet. One of the two usable addresses can be designated as the WAN IP on a local router, connecting it to the ISP (and then adding one of your local subnet IP's to it on the LAN side), and then advertise this as your "Way Out". The other IP can be used as a DMZ address, say for a web or mail server that's visible to the world. –  George Erhard Feb 27 '13 at 23:30

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.

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Worth noting the "technically usable" address is likely the internal address on the modem/router for purposes of routing traffic... if you have a "smart" router, you could serve on the host router. Probably a bad idea though. My ISP sends traffic via a routed CIDR block, so I could host stuff via my router... currently only do some SSH tunneling via my router. –  Tracker1 Feb 27 '13 at 22:29
    
Thank you for the detailed explanation.. Just for the clarification.. Do you want me to use 'IP subnet zero' command to use the Network ID as a usable IP??? –  user162140 Feb 28 '13 at 6:56
    
No, do not use if (repeating the list if circumstances listed at the end of my post) unless you have full control over every piece of hardware and over the OS. If you connect it to your ISP then you do not have full control over everything. I added that for completeness sake, but there are a lot of pitfalls you need to be aware of. It comes down to 'should work according to specifications` but also does NOT work in practise. It is nice background knowledge. If it usable in a test environment. but do NOT use it in a live/work situation. –  Hennes Feb 28 '13 at 15:37

It means that you will get a subnet with network mask 255.255.255.252. (or /30 in CIDR notation). You will have 4 addresses total, first containing only zeros (network address) and last containing 1's (broadcast address) - 00,01,10,11. Two other adresses can be assigned to hosts.

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Thanks a lot :) –  user162140 Feb 27 '13 at 18:22

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