Nat using network address?

Unanswered Question
Aug 18th, 2010


I have found the attached pdf in cisco documentation for CCNA4 Exploration. I don't understand how can this be right. How can I use the network or broadcast address for nat mapping? Please explain me how this works.

I have this problem too.
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kyukim Wed, 08/18/2010 - 08:42


1. ACL is defining which sourced traffic can be NAT translated.

2. ip nat pool defines local global addresses.

So,  Traffic sourced from, 192.168.20/24, and will be local inside addresses and will be translated to one of local global address of NAT pool in "" ~ "130".

It is is PAT with overload command.

NAT will start use TCP/UDP available ports with and move to after it consumes all available ports of

R2(config)#ip access-list standard R2NAT

R2(config-std-nacl)# permit

R2(config-std-nacl)# permit

R2(config-std-nacl)# permit

R2(config)#ip nat pool R2POOL netmask

R2(config)#ip nat inside source list R2NAT pool R2POOL overload


tmtudy999i Wed, 08/18/2010 - 10:06

Well it's not quite like this. On Gns it starts translating at .129 and only uses .129 and .130. This was my actual problem is a network address not a valid ip address. so how can nat translate into a network address?

If I force a static NAT using .128 it actually works. I  don't understand why but it seems that the ip route command  (ISP(config)#ip route serial0/0/0 from  that pdf) doesn't care if the routed ip address is a network/broadcast ip.

It seems that from a /30 range I can use not two but all 4 addresses if I use 2 static translations. Am I correct?

kyukim Wed, 08/18/2010 - 11:26


By default, "ip classless" is configured on IOS router and not shown from sh run. 

With ip classless enabled, you can use all 4 IP on /30 subnet. (or, you can say all IP address in any subnet.)

Here is a good document about it.

Enable Classless Routing Behavior

At times, a router might receive packets destined for a subnet of a network that has no network default route. a router in network connected to subnets,, and Suppose the host sends a packet to By default, if the router receives a packet destined for a subnet it does not recognize, the router discards the packet.

Figure 2     No IP Classless Routing

In, classless routing is enabled in the router. Therefore, when the host sends a packet to, instead of discarding the packet, the router forwards the packet to the best supernet route.

Figure 3     IP Classless Routing

To have the Cisco IOS software forward packets destined for unrecognized subnets to the best supernet route possible, perform the following task in global configuration mode:


Enable classless routing behavior.

ip classless


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