Many parts to discuss with linking cdjs and mixers together. Basically this is the process of connecting your cdjs and mixer to a network via ethernet cables. There are several features and advantages to doing this. I'll take it one section at a time.
Option 1: Linking 2 cdjs together with 1 Ethernet cable
This is the simplest way to create the network is to plug in one Ethernet (cat5) cable into each CDJ. Models that support this are the CDJ 900, 2000, 900NXS, 2000NXS and 2000NSX2. This allows you to plug in one USB into one CDJ. Then press the "LINK" option on the other CDJ. This will display all the contents of the USB on the 2nd CDJ and you can play tracks over the "linked network".
Option 2: Linking 2 or more CDJS via Ethernet Router/Switch/Hub
This method you will need to connect each CDJ with a separate ethernet cable to your router/switch/hub. This will also allow you to connect the mixer to the router for additional features. If you use a wireless router, this also unlocks additional features with rekordbox and phones/tablets. You can also connect a laptop via ethernet and use the Rekordbox function on the CDJS that support that option, such as the 900NXS or 2000NXS.
Tag DJ Sets:
Linking provides some more flexibility when tagging with another DJ. The first DJ can load a USB into deck A and then on deck B can click on LINK. Then the 2nd DJ can put a UBS into deck B and click LINK on deck A. I'll admit, it can get a bit confusing, but it does allow for pretty seamless tag sets and not having to load out USB sticks while tunes are playing, possibly ejecting the wrong one, etc.
Suggestions / Troubleshooting:
I suggest to always have a back up USB in each deck, just in case there is an issue with LINK status. If you see a message like "EMERGENCY LOOP" on one of the LINKED cdjs, it usually means that there some sort of interruption in the network. Could be a bad network cable, or the cable is loose or unplugged. Always bring back up ethernet cables as well.