• This has thread has somewhat become my de facto networking thread.

    Using nmap, why can I only see MAC addresses for devices on the same subnet?

    Do I need to add / remove a router firewall rule?

  • Using any tool you can only see devices on the same subnet.

    The subnet is the address plus a mask, and only things within the masked range are available to be routed to.

    i.e. if you have DHCP on 192.168.1.10/24 the /24 limits the routing table to 192.168.1.* so you can only see things on that subnet. (255.255.255.0 is a mask representing a /24, and more visually shows the 0 bit is the wildcard that you can route everything to and from).

    This is networking stuff... and there is more... like you can have multiple routers all include differing subnet masks for the same address range... in these cases the most specific (smallest subnet) wins... which all shows why your netmask cannot be 0.0.0.0 as it is least specific and is kinda meaningless.

    So... in your local network, the router manages the routing table and has knowledge of the netmask which defines the subnet, and that limits the routing within the router to that subnet. A client, whether via DHCP or static IP, belongs to the subnet from the routers perspective, and so will only see things within the subnet.

    How I configure my network: I have a single netmask for my DHCP and static IPs, and I apply that to both the WiFi and wired network... this means my wired desktop can actually see my wireless ChromeCast and other interesting things like that. It's one logical network even though I actually have 2 physical networks (the LAN and WiFi), because what makes it one logical network is the shared subnet that spans it all.

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