From Half-Life Library

SteamIDs as seen in the console using the status command

SteamID, short for Steam Identification is a number assigned to Steam accounts for the purpose of banning and/or tracking specific players that goes beyond their in-game name or Steam name. It is a unique numerical value that has at least 6 numbers but can reach up to 8 or more depending on how old the account is. Newer accounts will be given a higher number, where as, older steam users will have a lower value.

You may see player's SteamIDs as well as yours by typing "status" in the console while in or out of a game. The format will be as follows: STEAM_0:1:1234567

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