What's changed: Initial version
4.1NAT and PAT
Covers the role of NAT in translating internal private addresses into global addresses, choosing among one-to-one static NAT, pool-based dynamic NAT, and PAT (overload) that collapses many hosts to one address using port numbers, the inside local / inside global terminology, and the judgment skill of isolating a fault from a mislabeled ip nat inside / ip nat outside or from the translation table.
Global IPv4 addresses are exhausted, and assigning an individual public address to every device inside a company is impractical. Internally, therefore, we use RFC 1918 private addresses and translate them to public addresses via NAT on the border router that faces the internet. CCNA does not test rote "what is NAT"; it tests the practical judgment of which of static, dynamic, or PAT to choose for a given requirement (how many hosts exit through how many public addresses) and of isolating a fault from a misconfiguration or the translation table.
4.1.1NAT terminology (inside local / inside global)
- inside local is the address of an internal host as seen from inside (typically a private address, e.g.,
10.1.1.10). inside global is the address of that same internal host as seen from outside (the post-NAT public address, e.g.,203.0.113.10). Remembering "local = the inside viewpoint" and "global = the outside viewpoint" prevents confusion. - A precondition for NAT to work is that the internal-facing interface must carry
ip nat insideand the external (internet-facing) interface must carryip nat outside. If this inside/outside designation is swapped or omitted, no translation occurs at all even when the translation rules themselves are correct.
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