29 November 2010

NAT: Show Commands

NAT show Commands

The command most often used to troubleshoot NAT is show ip nat translations
Sample Output for the show ip nat translations Command
Router#show ip nat translations

Pro   Inside global         Inside local         Outside local     Outside global



Another useful command is show ip nat translations verbose 
Sample Output for the show ip nat translations verbose Command
Router#show ip nat translations verbose

Pro Inside global           Inside local        Outside local    Outside global


              create 00:00:02,       use 00:00:00,  flags:  extended


              create 00:01:13,       use 00:00:50,  flags:  extended

Here is an explanation of the additional fields in this command:
  • create- How long ago the entry was created (in hours:minutes:seconds).
  • use- How long ago the entry was last used (in hours:minutes:seconds).
  • flags- Indicates the type of translation. Possible flags are
    extended- Extended translation
    static- Static translation
    destination- Rotary translation
    outside- Outside translation
    timing out- Translation is no longer used due to a TCP FIN or RST

In general, the following is the terminology used in the show commands to display various addresses:
  • Inside local address- The IP address that is assigned to a host on the inside network. The address is probably not a legitimate IP address assigned by the Network Information Center (NIC) or service provider. This is typically an RFC 1918 address.
  • Inside global address- A legitimate IP address (assigned by the NIC or service provider) that represents (after translation) one or more inside local IP addresses to the outside world.
  • Outside local address- The IP address of an outside host as it appears to the inside network. Not necessarily a legitimate address, it can be allocated from address space routable on the inside.
  • Outside global address- The IP address assigned to a host on the outside network by the host's owner. The address was allocated from a globally routable address or network space.
show ip nat statistics is another useful command that gives the administrator information about NAT statistics. 
Sample Output for the show ip nat statistics Command
Router#show ip nat statistics

Total translations: 2 (0 static, 2 dynamic; 0 extended)

Outside interfaces: Serial0

Inside interfaces: Ethernet1

Hits: 135  Misses: 5

Expired translations: 2

Dynamic mappings:

-- Inside Source

access-list 1 pool net-208 refcount 2

 pool net-208: netmask

        start end

        type generic, total addresses 14, allocated 2 (14%), misses 0

To summarize, the two relevant NAT show commands are
  • show ip nat translations or show ip nat translations verbose
  • show ip nat statistics

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