No, PIX cannot block Skype. Skype has the capacity to negotiate dynamic ports, and to use encrypted traffic. With encrypted traffic, it's virtually impossible to detect it as there are no patterns to look for. You could eventually use a Cisco IPS (Intrusion Prevention System). It has some signatures that are able to detect a Windows Skype Client that connects to the Skype server to synchronize its version.
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...
[toc:faq]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 and UDP are I...