IP Address Assignment for the Virtual Interface

Applies To: Windows Server 2008, Windows Server 2008 R2

When a calling router initiates a connection, the router creates a virtual interface (sometimes called a virtual network adapter) and requests that the answering router assign an IP address to this virtual interface. The answering router then creates a virtual interface and requests an IP address for itself from the calling router. The virtual interface on the calling router connects to the virtual 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 virtual 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 do not conflict with any DHCP servers that might be present on your network, and that are not already statically 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, then you must create routes to the specified address ranges on the routers of your intranet so that traffic to the virtual 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 virtual interface, and requests that the answering router assign an IP address to this virtual interface, the answering router assigns the IP address specified in the calling router’s user account. The answering router then creates a virtual 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 2008 R2, Windows Server 2008, and Windows Server 2003 do not require a numbered connection. If one of the routers rejects the request for an IP address during the connection establishment process because no addresses are available to assign, probably due to a misconfiguration, a connection is still established. In this case, the virtual interface between routers does not have an assigned IP address. This is known as an unnumbered connection.