Network Address Translation Questions and Answers

Answer: C

Correct answer: 1, 3 and 5

The three methods are static, mapping one inside address to one global address permanently; dynamic, drawing from a pool as sessions arise; and overload, sharing one address by port. An IP NAT pool is a component used by the dynamic and overload methods rather than a method in itself, and double translation is not a Cisco NAT mode.

Answer: B

As in access-lists, you must configure your interfaces before NAT will provide any translations. On the inside networks you would use the command ip nat inside. On the outside interface, you will use the command ip nat outside.

Answer: A

Correct answer: show ip nat translations

This lists the translation table itself - each entry showing the inside local, inside global, outside local and outside global addresses, with ports where overloading is in use. It is the first command to run when NAT is suspected, since an empty table says the translation is not happening at all. show ip nat statistics gives totals and the interfaces involved rather than the entries.

Answer: D

NAT is not perfect, but there are some advantages. It conserves global addresses, which allow us to add millions of hosts to the Internet without "real" IP addresses. This provides flexibility in our corporate networks. NAT can also allow you to use the same subnet more than once in the same network without overlapping networks.

Answer: B

Correct answer: Outside local

The four terms are read as two questions: whose address is it, inside or outside, and which side of the translation is it seen from, local or global. The destination is an outside host, and before translation it is being referred to as the inside network sees it - hence outside local. In most deployments this is identical to the outside global address, since the outside host is not being translated.

Answer: A

Correct answer: Inside local

The inside local address is what the inside host is actually configured with - typically an RFC 1918 private address - before any translation occurs. Read the terms consistently: inside means the host belongs to your network, and local means the address as it is seen from inside it.

Answer: C

Correct answer: Inside global

This is the inside host’s address as it appears after translation, on the public side - the registered address the outside world sees. It is the counterpart of the inside local address, which is what the host actually carries, and the pair of them makes up each row of the translation table.

Answer: D

Correct answer: Outside global

After translation the destination is referred to by its address on the outside network - the real, registered address of the outside host. Together with the previous items this completes the set: inside local and inside global for your own host before and after, outside local and outside global for the far host before and after.

Answer: C

Correct answer: Check your interfaces for the correct configuration.

The commonest NAT fault is a missing or reversed ip nat inside or ip nat outside statement: without both sides marked, the router has no idea which direction to translate in and the table stays empty. Checking the interface designations, then the access list and pool, is the ordered way through it - and debug all on a production router is likely to overwhelm the CPU.

Answer: D

Correct answer: clear ip nat translations *

The asterisk clears every dynamic entry in the table, which is the usual way of forcing translations to be rebuilt after a configuration change - a stale entry will otherwise persist until its timeout. A specific entry can be cleared by giving its addresses instead, and static translations are removed by deleting their configuration rather than by this command.