How it works
Devices use a link-layer address to identify an interface locally and an IP address suited to their current network. Link-layer addresses may be assigned or randomized. The two address types serve different purposes and are not interchangeable.
Use the network address to reason about who can reach whom. Use the hardware address when the question is about a specific physical interface on the local segment.
Common error
Two devices with addresses that look similar are not necessarily on the same network. The mask decides that, not appearance.
Worked example
Decide whether two IPv4 clients share a subnet. Distinguish their IP and interface addresses.
Evidence
A: 10.8.4.10 / 255.255.255.0
B: 10.8.4.90 / 255.255.255.0
A interface MAC: 02:00:00:00:04:10The /24 mask places both IP addresses in 10.8.4.0/24. Their MAC addresses identify local interfaces, while IP addresses support network-layer delivery. A MAC address is not an internet route and may be locally administered or randomized.
Exercises
2.2.1
Are 10.8.4.250/24 and 10.8.5.5/24 in the same subnet? Give their network addresses.
Show solution
No. Their network addresses are 10.8.4.0/24 and 10.8.5.0/24, so communication requires routing between those networks.
2.2.2
A laptop moves to another IP subnet. Which address normally changes, and why is a MAC address not a substitute for the gateway setting?
Show solution
Its assigned IP configuration normally changes to fit the new subnet. A MAC address identifies an interface on a local link; the gateway setting identifies where off-subnet IP traffic should be sent.