Addresses and subnets: on my network, or through the router?

The idea

On a small office network, alice, bob and a printer share one wire with officeRouter, the router to a file server on another network. Alice reads her own prefix to decide: on my network, or through the router? Subnetting practice, one prefix at a time.

Steps

1. An address and its network

   Alice's IP address is 10.0.1.10/24, and the /24 says which addresses are her neighbours: every 10.0.1.x. Knowing exactly where her network starts and ends is what lets her decide. Ask a calculator what else it says.

   alice:~# ipcalc -npbm 10.0.1.10/24

2. Direct, or via the router

   The prefix matters because it picks the route a packet leaves by: straight to a neighbour, or to the router. Ask alice how she would reach bob (10.0.1.11), then the file server (10.0.2.10).

   alice:~# ip route get 10.0.1.11
   alice:~# ip route get 10.0.2.10

3. See the router in the path

   A decision in a table is only a plan; real traffic proves it. Traceroute lists every router a packet passes on its way. Trace the file server, then bob.

   alice:~# traceroute -n 10.0.2.10
   alice:~# traceroute -n 10.0.1.11

4. Prefix arithmetic

   A longer prefix means a smaller network, which is how one block is cut into several. Work out where the printer's 10.0.1.200 falls with a /26, then see /25 and /30.

   alice:~# ipcalc -nbm 10.0.1.200/26
   alice:~# ipcalc -nbm 10.0.1.200/25
   alice:~# ipcalc -nbm 10.0.1.200/30

5. A prefix too long: the detour

   A wrong prefix is a common typo, and its symptom is subtle. Break it: give alice /25 instead of /24, then trace the printer, 10.0.1.200, on her own wire.

   alice:~# ip addr add 10.0.1.10/25 dev eth0
   alice:~# ip addr del 10.0.1.10/24 dev eth0
   alice:~# traceroute -n 10.0.1.200

6. A prefix too short

   Now the other way, the typo that hurts more: give alice the same address with /16, and ping the file server. Then find the fix yourself.

   alice:~# ip addr add 10.0.1.10/16 dev eth0
   alice:~# ping -c2 10.0.2.10
   alice:~# ip addr del 10.0.1.10/16 dev eth0
   alice:~# ping -c1 -W2 10.0.2.10

Run these steps on real machines