Translate an address in both directions
01Basic Network Address Translation (NAT) maps a private IPv4 address to an outside address, then reverses that mapping for return traffic.
A reply addressed to 198.51.100.7:3017 reaches the same translation state and is rewritten to 10.0.0.10:3017. No extra tunnel header is added.
Source for this drawing: RFC 3022 · Read RFC section 2.1 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Basic NAT changes addresses; the chosen transport ports remain the same.
- RFC 3022 describes traditional NAT as predominantly outbound sessions from a private address realm. A realm is a network in which its addresses have meaning.
- Basic NAT binds a private address to an outside address. Several sessions from the same private host can share that address binding; simultaneous mapped hosts are limited by the available outside address pool.
- An outbound packet changes its source IP address. Return traffic changes its destination IP address back to the private host. TCP/UDP checksums also need correction because they cover IP addresses through a pseudo-header.
- Static address mappings can exceptionally permit outside-initiated access to selected internal hosts. Translation and permission to pass traffic are separate questions.
This is an Informational January 2001 publication, not an Internet Standard or a new RFC 2119 conformance profile. Its ordinary must/should wording is kept in context. All 198.51.100.0/24 and 203.0.113.0/24 addresses here are documentation stand-ins for outside addresses, not real globally routable allocations. IPv4 forwarding, TTL and routing policy are separate from the byte-rewrite helper.
Read the RFC section ↗