erc-autojoin-domain-only

erc-autojoin-domain-only is a variable defined in `erc-join.el'.
Its value is

t

Documentation:
Truncate host name to the domain name when joining a server.
If non-nil, and a channel on the server a.b.c is joined, then
only b.c is used as the server for `erc-autojoin-channels-alist'.
This is important for networks that redirect you to other
servers, presumably in the same domain.

You can customize this variable.