8. IP Routing

Question 1

How does NAT mitigate the exhaustion of IPv4 addresses?

Answer 1

NAT translates public IP addresses into private ones which don’t appear on the public internet. The NAT router will change incoming destinations and outgoing message’s sources using it’s translation table.

Question 2

What is the purpose of a routing table?

Answer 2

Routing tables are used to select a nex-hop router as the next destination for a datagram. When several destinations are matching, the route with the longest subnet mask is chosen, because it is more specific.

Question 3

What is OSPF?

Answer 3

Open shortest path first. Each node has a set of link state packets, one for each interface. Intially a router knows what it is directly connected to, but nothing else. A router sends it’s link state packets to all other routers, and receives theirs. A forwarding table is constructed for the network area.

Question 4

What are the 4 steps in OSPFs use of Dijkstra’s shortest route?

Answer 4

1. Initialise confirmed list with self entry.
2. Select the node just added to the c-list, call it next and examine it’s link state packet.
3. for each neighbour of next, calculate cost from next to neighbour
   a) if neighbour is not on c-list or t-list add (neighbour, cost, next-hop) to t-list
   b) if neighbour is on t-list, and cost is less than the currently listed cost for neighbour, then replace the current entry with (neighbour, cost, next-hop)
4. if the t-list is empty, stop. otherwise pick lowest cost entry from t-list, move it to c-list, and go to step 2.

Question 5

What 3 things have prolonged the life of IPv4?

Answer 5

• Classless Internet Domain Routing (CIDR)
• Some class A addresses were returned to IANA
• Network Address Translation (NAT)

Question 6

Why is NAT considered to be a “hack”?

Answer 6

• routers should only process up to layer 3
• violates the end-to-end argument
• problem should be solved by moving to IPv6, not keeping IPv4 alive

Question 7

How does a client connect to a server with a local ip address that is behind a NAT router?

Answer 7

The NAT router forwards connections to the server if the connection is at a given port.

Question 8

How can 2 NATed nodes call each other?

Answer 8

Using a relay, which bridges packets between the two nodes.

Question 9

What is ICMP?

Answer 9

Internet Control Message Protocol. Sends error and control messages, as a mandatory part of IP. e.g. time exceeded on datagram, notifies sender of TTL expiry