To implement IP filters at the headquarters, add source filters for the Boston distribution center for UDP port 1701 and IP protocol 50. Also, add destination filters for the headquarters for UDP port 1701 and IP protocol 50. The Windows 2000 Router service provides routing services in the LAN and WAN environments, and over the Internet, using secure virtual private network (VPN) connections. The VPN connections are based on the Point-to-Point Tunneling Protocol (PPTP) and the Layer 2 Tunneling Protocol (L2TP) L2TP is very similar to PPTP but uses UDP, and therefore can be used over asynchronous transfer mode (ATM), Frame Relay, and X.25 networks as well. When L2TP is used over IP networks, it uses a UDP port 1701 packet format for both a control channel and a data channel. L2TP can also be used with IPSec to provide a fully secured network link. Further, IP packet filtering provides an ability to restrict the traffic into and out of each interface. Packet filtering is based on filters defined by the values of source and destination IP addresses, TCP, and UDP port numbers, and IP protocol numbers. Inbound filters that are applied to the receiving traffic allow the receiving computer to match the traffic with the IP Filter List for the source IP address. Similarly, the outbound filters that are applied to the traffic leaving a computer towards a destination trigger a security negotiation for the destination IP address. That is why, to implement the IP filtering at the headquarters, you have to add a source address for the filters at the Boston center and a destination address for the filters at the headquarters.