Cannot seem to run boot.ipxe script

[NiKiZe]

you are only typing "http://172.16.1.251/boot.ipxe" ?
ipxe requires commands and don't know what to do with that string
but if you add chain before it, so that you get
"chain http://172.16.1.251/boot.ipxe" you should be good in terms of it being able to parse the line.

you might need to type dhcp first as well.
Also you might want to look at http://ipxe.org/embed