Parsing IP to extract subnet

That depends somewhat on what you intend to do with the information. For example, if you want to choose a conditional branch based on subnet, you can do something like this:

when CLIENT_ACCEPTED {
    if { [IP::addr [IP::client_addr] equals "192.168.0.0/16" } {
        pool pool-internal
    }
}

If you're simply trying to extract dotted-quad octets, then

scan