for example: if the AC source IP is 192.168.100.1:
option 43 hex 030d3139322e3136382e3130302e31 //192.168.100.1
the value 03 is fixed, 0d means the IP address length, 31 is the hex value of 1.
the more details you can check as fellow:
http://www.cisco.com/c/en/us/td/docs/wireless/access_point/1550/installation/guide/1550hig/1550_axf.html