CIDR entries and routing tables

Question

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?

Answer 1

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

Answer 2

Well, the most specific subnet is always used first:

/25 is more specific then /24. In your example, this would mean:

1. interface 0
2. Router 2
3. interface 1
4. Router 3
5. Router 4