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network-programmingcidr

CIDR entries and routing tables

I have a question which asks the following:

Suppose a router has the following CIDR entries in its routing table:

Net/Prefix Next Hop
128.96.39.0/24 Interface 0
128.96.39.128/25 Interface 1
128.96.40.0/25 Router 2
192.4.153.0/26 Router 3
default Router 4

For each of the following IP addresses, what does the router do if a packet with that address arrives?

  1. 128.96.39.10;
  2. 128.96.40.12;
  3. 128.96.39.151;
  4. 192.4.153.17;
  5. 192.4.153.90.

I know that /24 means that 24 bits are assigned to the network address meaning that 8 bits are assigned to hosts. How do I get the subnet mask from the IP address represented in decimal and then from there determine where the IP would go?

Solution

  • The Router takes the route with the longest fitting subnet mask.

    (i) interface 0 (ii) Router 2 (iii) interface 1 (iv) Router 3 (v) Router 4