Computer networks provide connectivity between computing devices on local networks and/or remote networks.  A connection between two remote networks typically occurs through a communications network such as the Internet.  Communications over the Internet pass through a series of communication links and routers/switches, many of which are located within third-party networks that are also interconnected with the Internet.  Thus, communications between two remote networks across the Internet are typically “open” because neither the sender nor the receiver controls the path that the communications traffic travels from source to destination.  Rather, the path is controlled by the various providers and devices located on the Internet itself. 
The connectivity between remote networks can be used to allow a client host to utilize a network-accessible service provided by a remote server host.  Common network-accessible services include but are not limited to Finger, Echo, SMTP, Telnet, SSH, FTP, DNS, HTTP and HTTPS as well as those services utilizing IP Protocol 50, ICMP, TCP/IP and UDP. 
Because private networks are connected to an open system such as the Internet, security is of great concern when communicating with other private networks via network-accessible services.  Often, network administrators install firewall devices, intrusion detection devices, intrusion prevention devices and/or other buffers on a local area network (“LAN”) in order to screen and/or filter incoming and/or outgoing content from the local network.  Alternatively, system administrators can install firewall, intrusion detection, or intrusion prevention software on a local host within a LAN in order to screen and/or filter incoming and/or outgoing content from the local host.  This can require the network administrator and/or the system administrator to install complex and costly equipment and/or software in order to ensure that the local network and/or local host is protected. 
Exemplary network security systems are described in U.S. Pat.  No.  6,687,831 to Albaugh et al.;  U.S. Pat.  No.  6,654,882 to Froutan et al.;  U.S. Pat.  No.  6,321,336 to Applegate et al.;  U.S. Pat.  No.  5,892,903 to Klaus; and U.S. Pat.  No.  5,557,752 to Smaha et al.  However, each of these network security systems requires the installation of costly and/or complex equipment and/or software. 
What are needed are improved methods and systems for inhibiting intrusion attempts directed at network-accessible services.