The 'jls' command gives a listing of the jails currently running on the system:

blue# jls
   JID  IP Address      Hostname                      Path
    19  10.10.4.107   druprod.example.com             /usr/jails/j/druprod
    18  10.10.4.102   ldap.example.com                /usr/jails/j/ldap
    17  10.10.4.110   ox.example.com                  /usr/jails/j/ox
    13  10.10.4.105   dns.example.com                 /usr/jails/j/dns
    12  10.10.4.104   sqltest.initech.com             /usr/jails/j/sqltest
    10  10.10.4.101   vhs.example.com                 /usr/jails/j/vhs
     3  10.10.4.106   www.example.com                 /usr/jails/j/www
     1  10.10.4.103   sql.initech.com                 /usr/jails/j/sql

You can use the 'jk' command to enter the jail based on matching the line
from the 'jls' output. It will inform you if there is more than one match:

blue# jk sql
   JID  IP Address      Hostname                      Path
     1  10.10.4.103   sql.initech.com                 /usr/jails/j/sql
    12  10.10.4.104   sqltest.initech.com             /usr/jails/j/sqltest
2 matches.
#

Specify a unique identifier to enter the jail:

blue# jk sqlt
Entering jail {sqltest.initech.com, 10.10.4.104, /bin/csh}
sqltest:blue#

Alternatively, you can specify a JID:

blue# jk -1
Entering jail {sql.initech.com, 10.10.4.103, /bin/csh}
sql:blue#