erc-common-server-suffixes

erc-common-server-suffixes is a variable defined in `erc.el'.
Its value is

(("openprojects.net$" . "OPN")
 ("freenode.net$" . "freenode")
 ("oftc.net$" . "OFTC"))

Documentation:
Alist of common server name suffixes.
This variable is used in mode-line display to save screen
real estate. Set it to nil if you want to avoid changing
displayed hostnames.

You can customize this variable.