What server or network are you trying to connect to? For instance, if you were trying to connect to the QuakeNet: Random UK server, in your Status Window, you would type:

/dns uk.quakenet.org
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* Looking up uk.quakenet.org
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* Resolved uk.quakenet.org to 213.221.165.248
-

If you get * Unable to resolve uk.quakenet.org, then your DNS server isn't working. Contact your ISP to correct that problem.

If you get the resolved IP, like 213.221.165.248, then try /server 213.221.165.248

Here are a couple of IPs for you to try for different network servers:

EFnet: 193.163.220.3 (irc.inet.tele.dk)
EFnet: 208.178.231.190 (irc.prison.net)
EFnet: 206.167.75.78 (irc.Qeast.net)

QuakeNet: 204.254.121.10 (whole.mi.us.quakenet.org)
QuakeNet: 213.221.165.248 (b0rk.uk.quakenet.org)

undernet: 129.27.9.248 (graz.at.Eu.UnderNet.org)
undernet: 132.207.4.32 (montreal.qu.ca.undernet.org)