Theoretical Question

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This is just a theoretical question so I can file it away in the memory banks for future reference.

You have a class C IP address allocation and you realise that you don't have enough IP addresses left to assign, what procedure do you go through to obtain another block of IP addresses? I heard that you have to apply to some sort of agency for a new block.

Just need a definitive guide for future reference.

 

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You could always subnet your subnet!

Usually you would apply to your ISP, however the way that things work these days is that everything resides behind NAT so you can use whatever IP Addressing scheme you like and you only have one public IP address that goes outside. Even that adress may them be NATed at the ISP.

 

So could basically have 2 sets of IP addresses behind NAT?

 

You could have as many different sets of addresses as you like!

this should provide you with a basic overview of how NAT works.

 

Thanks for the link simon, It just dawned on me that computers on your private network don't have to communicate with each other anyway, so it wouldn't really matter how many internal IP addresses you use, am I correct in my thinking?

 

If they don't communicate with each other then you don't have a network!

 

I'm thinking about it in the terms of my employer's network, where we have winterms that link to a switch which in turn links to a router and the servers are based at an office in another area, so they don't directly communicate with each other behind the private network.

 

Yes, but they are communicating, not directly with each other, but with the switch and the server. It sounds to me like what you are refering to are different subnets of a network, with the router acting as the bridge between the two.

It's difficult to explain without the aid of diagrams, if I get the time I'll go googling and see if I can find something that will help explain.

 

Yeah thats what I'm referring to so you would have to use an internal router to bridge the two subnets then that router would communicate with the other router that in turn communicates with the remote router where the servers are? Is that correct?

 

Inorder to try and keep this simple, yes you are thinking along the right lines, but it could get a whole lot more complex (VLANS) or even be far simpler. But to sum up nicely you are thinking along the correct lines. I'll leave it at that before I start confusing myself!

 

Ok so how would you connect these 2 routers together?

 

I think we're moving way beyond the scope of the original question here, but the sam way that you connect any two computers together - via a cable. If they are in the same physical building then it may be by Ethernet, or serial. Moving to different physical buildings you would have a leased line, or use a third party supplier and connect using something like Frame Relay (serial connector), ISDN, or perhaps fibre.

 

Answering the original question :-

1) Find out where the current /24 came from (i.e. - who is your supplier)
2) Ask if the block can be expanded, or if it has to be a new block. Some people find it easier to get a whole new block and renumber the network.
3) Justify the use of the block to the supplier.

If you now have non-adjacent blocks then you need a router to connect them.

Everything else depends on detailed network topology.

Harry.

 

So you install a brand new network at work and you haven't been assigned any IP addresses for the network who would you get the IP block from?

I take it, that the procedure for a home network is different?

 

Depends on whether this network needs access to the Internet or not.

If not - then assign a block from the private ranges.

If you need Internet access then ask your ISP. However - you should ask yourself if in fact NAT plus a private range will do what you want, as many ISPs will need convincing if you want a block bigger than about a /26.

No - exactly the same.

Harry.

 

The procedure would be exactly the same as if you were building a home network.

At home when you connect to the internet you connect to your ISP in a business network it is exactly the same. Your ISP allocates you an IP Address and you use it as your 'public' address.

Generally these days you only have a single 'public' address for each company, unless your company is huge.

At the entrance to your network, known as the Gateway you will have a router, or at home a DSL modem or simillar. On this gateway you will have an external interface that connects to the phone sockect (at home) or elsewhere. You also will have an internal interface that connects to your computer, or into a switch (work).

Inside the router, or often a server you set up your addressing scheme and the router knows where to send the traffic.

Sorry rushed ending - work calls!

 

Okay but you'd only need 1 IP address externally as it would only be the router that is connecting to the internet?

But internally you could use a B range IP address without having to apply for new IP addresses?

 

Correct.

Internaly you could use as much as you like from one (or more) of the three private ranges. The whole point of the private ranges is that they do not require formal allocation - you do your own thing. The downside is that they are not routable on the Internet. It is NAT that handles this.

Harry.

 

Thanks for that Harry, cleared everything up for me now