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Community Member

Network Address Translation and subnets

I've configure a 1720 router to use the serial0 interface with Dial-on-Demand capabilities and set up NAT for things like telnet and ftp from outisde sources to internal machines.

I've succesfuly done this using 128.6.0.0/24 subnet for my internal network. The trouble I have is that if I try and change the subnet to say 128.0.30.0/24 or 128.30.0.0/24 (including routes and NAT configs), I can no longer telnet or ftp in from the outside.

My question is this, are there restrictions on what subnets you can use to do NAT? Why would one subnet work and not another?

Any help would be appreciated, thanks.

2 REPLIES
Bronze

Re: Network Address Translation and subnets

Subnet numbers should not matter. Assuming you modified your routes and NAT accordingly, you might also want to clear your NAT transactions. Or see if rebooting

makes a difference. If not , open up a tac case.

~Zulfi

Community Member

Re: Network Address Translation and subnets

You should use RFC 1918 networks internally: 10/8, 172.16/12, 0r 192.168/16.

NAT is just exchanging of source and/or destination address. PAT alo remaps/mux ports. Exchanging means while a packet pathes the router, it reads and exchanges the coresponding header fields, like src or dst IP addresses. Which IP addresses should not matter at all.

Make sure that the public NAT address you are using is routed to you by your provider.

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