Prefix on Nat Pool

Cyanic

As in

ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 24

What is it for?

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Comments

thenjduke

It is the same thing that the netmask does. It identifies what network the pool addresses belong too.

burbankmarc

You can use the CIDR /nn instead of specifying a netmask.

Cyanic

Yes, I get that, but it seems useless in this case. I mean you specify the the start and stop address so what does it matter what the prefix is?

For instance these all do the same thing right?

ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 24
ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 25
ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 26
ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 27

burbankmarc

I don't know. I thought it'd have something to do with the routing table or CEF or something but I just threw a config together and it didn't affect that stuff at all.

notgoing2fail

Cyanic wrote: »

Yes, I get that, but it seems useless in this case. I mean you specify the the start and stop address so what does it matter what the prefix is?

For instance these all do the same thing right?

ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 24
ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 25
ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 26
ip nat pool ovrld 172.16.10.1 172.16.10.1 prefix 27

hmmm.....not you've got me thinking.....

fly351

Cyanic wrote: »

Yes, I get that, but it seems useless in this case. I mean you specify the the start and stop address so what does it matter what the prefix is?

Just off the top of my head... if your NAT'ing to the internet, wouldn't it be useful to know the network/broadcast addresses of the subnet you are on?

Cyanic

fly351 wrote: »

Just off the top of my head... if your NAT'ing to the internet, wouldn't it be useful to know the network/broadcast addresses of the subnet you are on?

Yes and those are defined on the interface itself. The NAT pool simply defines a pool of addresses to use for the translation. Since you define the start and stop points of that, it just seems like the prefix is unnecessary. There may be some use for it that I am not aware of though.

stuh84

At a guess, if the address has a prefix, it could then be filtered with a prefix-list. I'm not sure if thats definitely the case, but its possible I guess.

notgoing2fail

Cyanic wrote: »

Yes and those are defined on the interface itself. The NAT pool simply defines a pool of addresses to use for the translation. Since you define the start and stop points of that, it just seems like the prefix is unnecessary. There may be some use for it that I am not aware of though.

Here you go!

The required netmask (or prefix) parameter performs a kind of verification check on the range of addresses. If the address range would not be in the same subnet assuming the configured netmask was used, then IOS will reject the IP NAT POOL command.

Cyanic

notgoing2fail wrote: »

Here you go!

The required netmask (or prefix) parameter performs a kind of verification check on the range of addresses. If the address range would not be in the same subnet assuming the configured netmask was used, then IOS will reject the IP NAT POOL command.

Nice find. Still odd in my book, but good to know.

notgoing2fail

Cyanic wrote: »

Nice find. Still odd in my book, but good to know.

Well it's very interesting, I suppose the range has to be within a subnet. Sounds like it can't crossover to another network.