IPv4 — end of module

What this checks. That you can perform a forwarding decision by hand and say what IP does and does not promise. You need 3 of 4 to pass, and every option is explained afterwards — including the ones you did not pick.

1. 1/4A router's table holds 10.0.0.0/8 via A, 10.0.20.0/24 via B, and 0.0.0.0/0 via C. Where does a packet for 10.0.20.30 go?

   Work out a value

   Via B

   Via A, because /8 was configured first

   Via C, because a default route always applies
2. 2/4Which IPv4 header field exists to stop a packet circulating forever?

   Match a field to its purpose

   TTL

   Identification

   Header checksum

   Protocol
3. 3/4What is the difference between the RIB and the FIB?

   Tell two ideas apart

   The RIB holds every route offered, winners and losers; the FIB holds the ones actually used to forward

   The RIB is on the control plane's CPU and the FIB is in hardware, and nothing else differs

   The FIB holds static routes and the RIB holds learned ones
4. 4/4Someone says 'the header checksum means IP delivers data intact'. Why is that wrong?

   Say why an answer is wrong

   The IPv4 checksum covers the header only — the payload is not checked at all

   The checksum is optional in IPv4

   Routers do not verify it