27018 MongoDB-shard TCP
Reference for TCP port 27018, commonly associated with MongoDB-shard.
What it's for
Port 27018 is the conventional port for a MongoDB shard server within a sharded cluster deployment. In a sharded MongoDB setup, data is horizontally partitioned across multiple shard servers, and mongos router processes (listening on the standard 27017) direct each query to the appropriate shard(s) behind the scenes — application clients typically never connect to 27018 directly.
Protocol basics
Communication with a shard server uses the same BSON-based MongoDB wire protocol as the standard port 27017, just on a different conventional port to distinguish shard-level traffic from the client-facing router layer in a multi-component cluster. Because shard servers hold actual application data, they warrant the same network isolation and authentication requirements as a standalone MongoDB instance — access should be restricted to the cluster's own internal components, not exposed for direct external connections.