Relay
The relay can be used to allow different subnetworks to send network messages to each other, without exposing components to computers in other networks. Keeping components local is usually a good idea, to avoid computers using the wrong screen or to avoid component overflows to happen (causing computers to crash and refuse to boot up).
The relay can be upgraded by inserting a wireless network card to also relay messages wirelessly. Wireless messages can be received and relayed by other relays with a wireless network card, or by computers with a wireless network card.
Alternatively the relay can be upgraded using linked cards. In this case it will forward messages through the tunnel provided by the linked card, too; at the usual cost, so make sure the relay is sufficiently powered.
Relays do not keep track of which packets they forwarded recently, so avoid cycles in your network or you may receive the same packet multiple times. Due to the limited buffer size of relays, sending messages too frequently will result in packet loss. You can upgrade your relays to increase the speed with which they relay messages, as well as their internal message queue size.
Packets are only re-sent a certain number of times, so chaining an arbitrary number of relays is not possible. By default, a packet will be re-sent up to five times.