About NAT address allocation

[ddujma]

Hi dear friends,

As as was studying cisco materials I began packet tracer activity.

There was an task where it states that I'm given three public IP addresses for my NAT configuration: 209.165.202.128 - 130. The connection between ISP and my router is an serial connection with a pre-configured IP of 209.165.200.224/27.

Q: if the ISP who gave me that IP range wants to route packets to my network it would state a route, and could give it a /30 mask to make the most precise configuration for the given range. Is that correct?

If that is correct, than I would possibly be able to have four IP addresses 128 - 131. What happens to the 131, if i haven't bought it from the ISP. Do they loose the "one precious address" or I got something wrong.

And, if i didn't have preconfigured IP on my serial link, does that mean in the case i got /30 subnet, that I could only use one IP address. They are a bit thin in explanations about NAT.

Thanks in advance wise one's ;)

[Fuzz]

With a /30 mask, you only have two host addresses. The first address is the subnet address, the last address is the broadcast address. So you only have 209.165.202.129 - 130 to assign to an interface.

If the mask of the ISP default gateway is /29 or lower, then you would only have access to the three addresses assigned to you. The others would be assigned to other clients of the ISP using the same default gateway.

With NAT you only need one address anyway, as PAT allows for some 65000 translations per address. Using more than one address combines PAT with dynamic addressing.

[ddujma]

Thanks for quick response, but I'll need you to further explain how do I only get three addresses from /29 range?

Isn't it four useful?
7 addresses - network_Add - broadcast_Add - ISP_sideAdd = 4, or i missed something? :confused:

And I agree that one address is enough using PAT, but for some enterprises or medium size business we often need at least one static for servers and so..

[Fuzz]

If an ISP assigns a /29 subnet to clients, they might allow only three addresses out of those available 6 to you, giving the other addresses to other clients. You dont get the whole /29 subnet, what you actually get is
209.165.202.129 /32, 209.165.202.130 /32 & 209.165.202.131 /32.

Remember the more subnets you create, the less host addresses are available to assign out of the entire address space. With the IPv4 addresses going to less than 8% left, an ISP might have subnets of 64 assigned to many different clients. In other words, you share the default gateway with many other organisations. This saves host addresses.

[ddujma]

It was a bit puzzling, especially when i think about different kinds of infrastructures such as DSL and Frame Relay, but I forgot that /32 mask could do the trick.

Thanks for reply,
Cheers.