Oh, I just ban everyone. I'm really quite antisocial.
I think you're talking about second-level domains, Al. IP addresses are not necessarily for just specific servers, but for specific machines. True, most services (dial-up, DSL, satellite, cable, etc.) use DCHP (i.e. your IP address will change, within a certain range, after awhile), but if someone's hitting you HARD with the spam, sometimes just banning the username isn't enough.
On rare occasions, I've had to ban entire subnets, which could mean banning 256 or more users (depending on what segment I'm starting with) or an entire second-level domain, but that's rare.
Ultimately, the idea behind IP banning is to make life more difficult for spammers and trolls. It's definitely not a cure-all, because if you banned my IP address, I could get another one in a couple minutes. Ban my username AND IP, it'll take me a few more minutes. Ban my entire subnet and username, still more, etc.
FWIW, if you're a known spammer and your IP is 1.2.3.4 (using IPv4, of course), chances are that if I ban 1.2.3.4, I'm not going to ever ban a legitimate user. If I ban 1.2.3.x, there's a greater chance, but doubtful, as most spammers seem to use the same subnets (so I've just banned 256 potential baddies). 1.2.x.x is much more risky and 1.x.x.x may cut me off from teh werld!!!111one
In any event, the service that Helen mentions isn't quite what we have here on the WF. Service #1 (Akismenet or something like that) checks the content of any post against a list of spam-target words and throws the post into "Moderated" where no one can see it, if the service is tripped. Service #2 is me and our Admins: we manually check the IP address, username and/or e-mail address of any new user against lists of known spammer details (Google will pop up hundreds of these services) and if they match, we ban the user and leave a message that says, "Known spambot username, IP address or e-mail address. Questions? Contact me at
thesaxinfo@gmail.com." Additionally, I do implement most of the security tweaks mentioned on the vBulletin websites.