Aparentemente, no IIS6, a configuração da interface do usuário está quebrada e isso precisa ser feito usando httpcfg da qual a essência é:
Type the following, where xxx.xxx.x.x is the IP address you want to add:
httpcfg set iplisten -i xxx.xxx.x.x
When this succeeds, Httpcfg returns the following: HttpSetServiceConfiguration completed with 0
To view additional status codes, see the Httpcfg help. After the IP address is added, use the following command to list it:
httpcfg query iplisten
Httpcfg returns the following: IP :xxx.xxx.x.x. Restart the service