IP Address Assignment for the Logical Interface

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

When a calling router initiates a connection, the router creates a temporary logical interface (also known as a virtual interface or a virtual network adapter) and requests that the answering router assign an IP address to this logical interface. The answering router then creates a logical interface and requests an IP address for itself from the calling router. The logical interface on the calling router connects to the logical interface on the answering router to form the demand-dial connection. The IP address assignment for each demand-dial interface lasts for the duration of the connection. These IP addresses can be either private or public IP addresses. If both requests are successful, the logical interface on the PPP connection for each router is assigned an IP address from the other router. This is known as a numbered connection. In the absence of a numbered connection, a site-to-site connection can also use an unnumbered connection.

Numbered Connection

A numbered IP address assignment can occur in one of the following ways:

  • Dynamic IP addresses allocated by a DHCP server. The answering router obtains the IP address to assign to a calling router from a DHCP server. This is the default method for IP address assignment. If no DHCP server is available, the router uses an address from the Automatic Private IP Addressing (APIPA) range 169.254.0.1–169.254.255.254. You must install a DHCP server on each network that contains an answering router.

  • Dynamic IP addresses allocated from a static address pool configured on the answering router. The answering router obtains the IP address from a static pool of addresses. If you configure a static address pool, be sure to use only IP addresses that are not in a range that your DHCP server might assign to another computer and that are not already assigned to specific computers. The pool can be ranges of addresses that are a subset of addresses from the IP network to which the server is attached or from a separate subnet. As described earlier, if the static IP address pool address ranges represent a different subnet, ensure that routes to the address ranges exist on the routers of your intranet so that traffic to the logical interface of a calling router is forwarded to the remote server.

  • Static IP address specified in the calling router user account. You configure a static IP address on the router user account Dial-in tab for both the calling and the answering router. In this case, when a calling router initiates a connection, creates a temporary logical interface, and requests that the answering router assign an IP address to this logical interface, the answering router assigns the IP address specified in the calling router’s user account. The answering router then creates a logical interface and requests an IP address for itself from the calling router. The calling router assigns the IP address specified in the answering router’s user account.

Unnumbered Connection

Site-to-site connections in Windows Server 2003 and Windows 2000 do not require a numbered connection (Windows NT 4.0 RRAS connections do require a numbered connection). If one of the routers rejects the request for an IP address during the connection establishment process between a calling and an answering router (because no addresses are available to assign, probably due to a misconfiguration), a connection is still established. In this case, the logical interface on the PPP connection does not have an assigned IP address. This is known as an unnumbered connection.

The routing protocols provided with Routing and Remote Access cannot operate over unnumbered connections. Therefore, you must use static routing if you use unnumbered connections.