Cisco’s NAT Terminology

Normally this is quite confusing but the following diagram should help to sort things out.

  • Inside local address—The IP address assigned to a host on the inside network. This is the address configured as a parameter of the computer OS or received via dynamic address allocation protocols such as DHCP. The address is likely not a legitimate IP address assigned by the Network Information Center (NIC) or service provider.
  • Inside global address—A legitimate IP address assigned by the NIC or service provider that represents 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 is allocated from an address space routable on the inside.
  • Outside global address—The IP address assigned to a host on the outside network by the host owner. The address is allocated from a globally routable address or network space.

Configuring NAT

Step 1 – Define which interfaces are inside or outside using

ip nat { inside | outside }

Step 2 – Configure your flavour of NAT

Inside local -> Inside gobal (Dynamic)

ip nat inside source list pool
the ACL defines the inside local addresses (RFC1918 generally)
the pool specifies the inside global addresses

Inside local -> Inside global (Static)
 ip nat inside source static


Outside Global -> Outside Local (Dynamic)
ip nat outside source list pool
the ACL defines the outside global addresses
the pool specifies the outside local addresses (RFC1918 generally)

Use can used to hide outside global addresses that may have already been used in your internal network.

Outside Global -> Outside Local (Static)
 ip nat outside source static

    Advertisements

    Leave a Reply

    Fill in your details below or click an icon to log in:

    WordPress.com Logo

    You are commenting using your WordPress.com account. Log Out / Change )

    Twitter picture

    You are commenting using your Twitter account. Log Out / Change )

    Facebook photo

    You are commenting using your Facebook account. Log Out / Change )

    Google+ photo

    You are commenting using your Google+ account. Log Out / Change )

    Connecting to %s