How do you generate those URLs that contain IP-Addresses? Could it be that they are contained e.g. in web content articles?
Also, check your settings, either in portal-ext.properties or in the database (e.g. through Control Panel/Server Administration. For example there's this setting that can be done in portal-ext.properties - maybe overridden in database:
#
# Set the hostname that will be used when the portlet generates URLs.
# Leaving this blank will mean the host is derived from the servlet
# container.
#
web.server.host=
If this doesn't solve it, you might have set a virtual host for the site in question or for the whole portal.
A quick way to backtrace might be to grep for the host name / ip address in a database dump and try to identify the relevance&origin of the hit(s)