NAT Irule

You can test how to get the last octet of the destination IP with something like this:

when RULE_INIT {
   set ::translated_ip_prefix "1.2.3."
   set destination_ip 10.20.30.40
   set last_octet [getfield $destination_ip "." 4]
   set translated_ip $::translated_ip_prefix$last_octet
   log local0. "\$translated_ip: $translated_ip"
}

(logs: $translated_ip: 1.2.3.40)

You could then use a rule like this on a fastL4 network VIP:

when RULE_INIT {
   set ::translated_ip_prefix "10.20.30."
}
when CLIENT_ACCEPTED {
   node $::translated_ip_prefix[getfield [IP::local_addr] "." 4] [TCP::local_port]
   log local0. "Translated IP: $::translated_ip_prefix[getfield [IP::local_addr] "." 4] [TCP::local_port]"
}

I didn't test this, but it passed a syntax check.

Aaron