ip filtering l4...

we know that extended acl's can filter on l4 information such as protocols...we know that l3 devices excel in the ip arena... so how does a l3 device interpret l4 (transport) information...

the l3 device uses the protocol field in the datagram to determine the 8 bit number to protocol association that was established by iana in rfc 1700, here: http://www.rfc-editor.org/rfc/std/std2.txt

the protocol field essentially describes the data content of the packet...