With replica set configuration using IPv6 addresses directly (not hostnames), when connecting to the replica set, the connection fails.

Example:

{
	"_id" : "myReplSet",
	"version" : 5,
	"protocolVersion" : 65,
	"members" : [
		{
			"_id" : 0,
			"host" : "2607:f160:10:401b:ce:9:0:2:27017"
		},
		{
			"_id" : 1,
			"host" : "2607:f160:10:401b:ce:9:0:1:27017",
			"votes" : 0,
			"priority" : 0
		}
	]
}

When connecting to mongodb://localhost:27017/myDb?replicaSet=myReplSet, it fails to connect. Previous versions worked.

The error is in
https://github.com/mongodb-js/mongodb-core/blob/v2.1.19/lib/topologies/replset.js#L328

host: _server.split(':')[0],
port: parseInt(_server.split(':')[1], 10)

which is splitting the string on the first ':' instead of the last.

simple fix:

host: _server.substring(0, _server.lastIndexOf(':')),
port: parseInt(_server.substring(_server.lastIndexOf(':')+1), 10)