How exactly do subnet masks work in terms of how packets are delivered?

Question

I’m studying for a certification at the moment so I know a medium amount about the structure of subletting. The one thing my courses don’t explain very well is the actual function/process of how this works.

Specifically one thing I learned really upended my understanding. The example was if you have a host with the address 10.1.16.42/24 and a host with the address 10.1.16.200/16 would they need a router to communicate?

From my understanding they would, the first host is on the network 10.1.16 and the host section is 42. The second one is on 10.1 and the host section is 16.200. Since their network sections are different why do they not need a router to communicate if their network id’s aren’t the same?

In: Technology

Answer 1

Host A has been told its address is 10.1.16.42 on a /24 subnet, so it thinks all 10.1.16.x addresses belong in its subnet and should be reachable “directly”. Host A thus thinks Host B (10.1.16.200) is in its subnet.

Host B has been told its address is 10.1.16.200 on a /16 subnet, so it thinks all 10.1.x.x addresses belong in its subnet and should be reachable “directly”. Host B thus thinks Host A (10.1.16.42) is in its subnet.

Each server has a different understanding of the subnet scope but they *do* agree that the other is a part of the same network, so they’re perfectly happy to send out packets to the local switch or wire.

Subnet masks are not part of the packet. The packet does not care; the packet just says “From: 10.1.16.42, To: 10.1.16.200, With Love.” The switch or wire does not care about subnets; higher-level subnet routing is not their problem* – the packet can either reach the exact destination machine or it can’t.

Remember that each host does not have a strong understanding of the larger network – they only know what they’ve been told about their own network config and routing table and they act accordingly, trusting that they haven’t been lied to. As long as they get good results from good-faith obedience to their own rules, they don’t tend to care what happens after that.

* We’re assuming for the sake of the example that the physical switch and network are very simple.