Think of jails as VMs without the overhead of having the same OS multiple times in memory. Similarly you can't use host's IP by any of the guests.
That said many people go around that by simply binding the jail against an unsused loopback address (127.0.0.0/8) and then use firewall such as pf to redirect specific ports to given jail, like here http://blog.burghardt.pl/2009/01/multiple-freebsd-jails-shar...
That said many people go around that by simply binding the jail against an unsused loopback address (127.0.0.0/8) and then use firewall such as pf to redirect specific ports to given jail, like here http://blog.burghardt.pl/2009/01/multiple-freebsd-jails-shar...