The PIX does not have the ability to perform deep packet inspection to the level needed to catch skype, only ASA has this ability. But on PIX you could try to block UDP port 1247 and TCP ports 2940-3000 but if these ports are blocked, then skype uses 443 to tunnel out of the network to talk to the SN's (supernodes). Unless you plan on blocking all SSL connections, this is really not a feasible fix.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...