Servers are powerful computers that players connect to in order to play Agar.io.
Regions
Servers are split into regions, which are:
The 3 Asia-Pacific servers (East Asia, China, Oceania) tend to be the hardest in difficulty for all game modes, as the average skill level of players in this region is usually higher. FFA/Experimental Mode in these servers are also full of slow-teams. The China server isn't even hosted in China, and all of these servers are hosted by Linode.
Technical Details
Agar.io uses a classical client-server model. Clients connect to a server. They then can send messages to the server and receive messages from the server - and vice versa. Clients do not communicate with other clients directly but always use the server as a „man in the middle“.
The server is responsible for calculating the game world. Changes to the state of the game can only be done by the server. Clients only send messages to the server that describe the input of the player. For example, if the player presses space bar to split, this information will be sent to the server and the server will actually check if splitting is possible and if it is, perform the split.
Servers are automatically provided; self-hosting of servers is not necessary. When a player presses the connect button there is a mechanism that automatically directs the player to a randomly chosen server.
If a player disconnects from a server, for example due to a temporary interruption of the internet connection, there is no way to reconnect and continuing to play with the given mass.
Server Closure
If a server has few players or has been online for a while, it may not let any new players join. Eventually all players could be disconnected without warning.
Agar.IO Private Servers
Agario has several third party mods that allow you to host your own private server. Citadel Servers allow you to rent a private Agar.io server from them so you can self-host with your own mods and ruleset.