Network Address Translation (NAT)
Motivation
Local network uses just one IP address as far as outside world is concerned:
- Range of addresses not needed from ISP
- Just one IP address for all devices
- Can change addresses of devices in local network without notifying outside world
- Can change ISP without changing addresses of devices in local network
- Devices inside local net not explicitly addressable
- Visible by outside world
Implementation
NAT router must:
- Outgoing datagrams: Replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)
- Remote clients/servers will respond using (NAT IP address, new port #) as destination addr
- Remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, ,new port #) translation pair
- Incoming datagrams: Replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
