Packet Tracer 4.1 models protocols not included in earlier versions. These protocols include models of HTTP, DNS, TFTP, Telnet, TCP, UDP, Single Area OSPF, DTP, VTP, and STP. Also, in many cases, existing models of IP, Ethernet, ARP, wireless, CDP, Frame Relay, PPP, HDLC, inter-VLAN routing, and ICMP have been extended. Models of Integrated Service Routers (ISRs), including the 2811, 1841, and Linksys WRT300N, have been added. Very simple models of cable modems, DSL modems, and IP Phones have also been added.