When I was configuring the access list that should prevent spammers from misusing my workstations, I obviously had to figure out the IP address of the ISP’s SMTP server (access lists and object groups accept IP addresses). I almost started nslookup on my Linux workstation, but then decided to try entering a hostname in an IOS ACL … and it works. Unfortunately, IOS performs a DNS lookup when you enter the hostname (assuming you have configured the ip name-server) and stores the resulting IP address in the ACL definition:
rtr(config)#ip access-list extended InsideList
rtr(config-ext-nacl)#permit tcp any host smtp.example.com eq smtp
Translating "smtp.example.com"...domain server (192.168.0.1) [OK]
rtr(config-ext-nacl)#do show access-list InsideList
Extended IP access list InsideList
10 permit tcp any host 192.168.2.3 eq smtp
You can enter hostnames in ACLs or network object groups. In both cases, the name is immediately translated into an IP address.