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Routing Tables

Lesson 14 of 32 in Coddy's The OSI Model course.

A router picks the next hop by consulting its routing table: a list of "for this address range, send it that way" rules. When several rules match, the most specific one wins: the longest prefix.

Consider a router with three routes:

0.0.0.0/0 via gateway A (the default: matches everything)
10.0.0.0/8 via gateway B
10.1.0.0/16 via gateway C

A packet to 10.1.2.3 matches all three, but /16 is the longest prefix, so it goes via C. This rule, longest prefix wins, is the entire logic of internet routing.

Challenge

Easy

Using the routing table from the theory, print the gateway letter each packet leaves through, one per line, in this order:

10.1.2.3
10.9.9.9
8.8.8.8

Expected output:

C
B
A

Try it yourself

Terminal

All lessons in The OSI Model

1Why Layers

The Layering Idea Seven Layers Numbers and Names Recap - The Stack

4The Network Layer

IP and TTL Ping and ICMPRouting TablesRecap - Layer Three

7Encapsulation

Wrapping Data Headers Everywhere Reading a Capture Recap - Follow the Wrap

2The Physical Layer

Bits on a Wire How Long Will It Take Duplex and Collisions

5Transport: TCP and UDP

Ports The TCP Handshake TCP vs UDP Sockets in Action Recap - Layer Four

8The Journey of a Packet

A Click Happens Crossing the Networks Arriving and Climbing Recap - End to End

3Data Link and MACs

Frames and MACs Reading a MAC ARP Recap - Layer Two

6The Upper Layers

Session and Presentation The Application Layer Watching the Layers Recap - Top of the Stack

Practice on your own: Terminal playground