IP Addressing and Subnets
Every device on a network has an IP address and a subnet mask, and together they answer a single question: who is this device allowed to talk to directly? The mask is what splits the address into a network part and a host part. A /24 mask, written 255.255.255.0, fixes the first three numbers as the network and leaves the last for individual hosts, giving 254 usable addresses. Two devices that share the same subnet — say 192.168.1.10 and 192.168.1.20 on a /24 — can reach each other directly. Put one of them on 192.168.2.x instead and they cannot, even sitting on the very same cable, because each one looks at the address and concludes the other is "somewhere else."
That word "elsewhere" is where the gateway comes in. A device only hands its traffic to the gateway — a router — when it has decided the destination lives on a different subnet. For a self-contained show network with no internet connection, you often need no gateway at all; one consistent subnet is enough for everything to find everything else.
AV systems almost always live in the private address ranges set aside by RFC 1918: the 10.x, 172.16-to-31.x, and 192.168.x blocks. And it is worth holding onto the single most common networking fault, because it is so undramatic: a device with the wrong IP or the wrong mask, physically plugged in and showing a link light, yet completely isolated in every way that matters.
Same subnet or no communication.