Port Address Translation(PAT):

• This is the most common type of NAT and its many to one mapping.
• Here multiple internal IP address can be mapped to single public IP address.
• In this case mapping is done on the combination of port number and IP address.
• Means the NAT router would replace the internal IP address as well as internal source port with public IP address and new port from pool of ports.
• Since there is a single public IP address, thus port number can be used to distinguish traffic that is which traffic belongs to which IP address.
• It is cost effective as only single public address need to be brought.
• It is one of the most effective way to overcome IPv4 address depletion.

How does this work:

• When a packet from internal host is received on the NAT router, it replaces the internal IP address with common public IP address and its source port from pool of ports.
• This mapping of internal IP address and port number to external IP address and port number is maintained in NAT table.
• Packet with new parameters(IP and Port) is send to external world and when response comes back with new parameter to the NAT router, table is scanned to check the presence of this mapping.
• If found, again the public IP address is translated along with the port number and response reaches to the correct recipient else packet is dropped.

There are four address terms used in NAT:

Inside Local:

This is the private IP address which is not seen to public world and needs translation. This is the IP address assigned to our internal workstation.

Example: 10.0.0.100

Inside Global:

This is the public IP address that is to which the internal IP address are mapped to. That is this IP address is of router interface which is facing the internet.

Example: 155.4.12.1

Outside Local:

This is the private IP address assigned to the end device at other network.

Example: 15.0.0.1

Outside Global:

This is the public IP address to which the outside private IP address are mapped to. That is this IP address is of the router interface which is facing the internet at the other side of network

Example: 155.4.12.4

Categories: Networking