http://tools.ietf.org/html/rfc3442 This is used to add a classless static route to the DHCP clients. To add option 121 to a Mikrotik DHCP server, it's value is specified in HEX. The format is as follows. 0xnnddddddddgggggggg where n=mask, d=destination, g=gateway. To convert ip address to HEX, you convert each octet, so 192=C0, 168=A8, 55=37, 1=01 You can use a tool such as http://www.miniwebtool.com/ip-address-to-hex-converter/?ip=192.168.55.1 Example: To add a route to the destination network of 192.168.55.0/24 via gateway 172.16.10.1. /ip dhcp-server option add name=classlessroutes code=121 value=0x18C0A837AC100A01 where 18 is 24 in hex. *note: depending on the subnet mask, you may only need to specify 0-4 octets. In fact only the non-zero, or network portion of the subnet. Here is a table from the RFC. subnet mask Number of octets 0 0 1- 8 1