Add your subnet addresses to the current (127.0.0.1) option and delimit using spaces.

# default: off
# description: SWAT is the Samba Web Admin Tool. Use swat \
#              to configure your Samba server. To use SWAT, \
#              connect to port 901 with your favorite web browser.
service swat
{
        disable = no
        port            = 901
        socket_type     = stream
        wait            = no
        only_from       = 127.0.0.1 10.11.12.0/24
        user            = root
        server          = /usr/sbin/swat
        log_on_failure  += USERID
}

Reload xinetd when you have done the edit

service xinetd reload