NAT Doubts

[eduangelo]

Hello guys,

I configured the internet output interface with the ip obtained through DHCP.

Now I want to configure static NAT to allow external access to the HTTP server.

But as the external address is the address obtained through DHCP.

There will be times when NAT will fail. Is there a way to resolve this situation?

[Alan Ng'ethe · Accepted solution]

Have you tried to use dynamic dns?

External clients coming in would reach out to the dns name of the web server, which would in turn be registered dynamically by the device each time it changes its ip address. I have not used this in practice and have no clue whether its free, but think its worth having a look at.

[Reza Sharifi · Accepted solution]

Hi,

That is not an uncommon setup for a small network.  The only time that NAT can fail is if yours or the provider's interface go down or if you switch to a different provider, at which time you would have to switch IP anyway. 

HTH

[Francesco Molino · Accepted solution]

Hi

You can do a nat for port forwarding without setting up an IP but setting it up with your interface. Then you can configure DDNS on your router to ensure you will always access your web server using this ddns fqdn.

[paul driver]

Hello

I agree with @Francesco Molino for nat internally to outside this can be accomplish by specifying the wan facing interface instead of any ip address for translation, but the problem will occur if your isp allocates another public ip address for your rtr then any external customer connecting to this internal web server will still try connect via the web servers FQDN so this need to be updated every time your wan rtrs ip address changes. -- DDNS is the way to go.

As for the NAT configuration - see example below:

access-list 1 permit 192.168.23.0 0.0.0.255 <  - internal lan subnet
ip nat inside source list 1 interface FastEthernet0/0 overload  <--dynamic nat for internal users
ip nat inside source static tcp 192.168.23.3 80 interface FastEthernet0/0 80  <  PAT for internal Web server http
ip route 0.0.0.0 0.0.0.0 <wan interface> dhcp/dialer etc..