Routing Technologies

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

Routing Technologies

Routing technologies manage the flow of data between network segments, which are also known as subnets. Windows Server 2003 includes several routing technologies, including demanddial routing, unicast Internet Protocol (IP) routing, IP multicasting, and network address translation (NAT) functionality, which are discussed in this subcollection.

Routing Components

This subcollection contains information about the following routing technologies:

  • Demand-dial routing

  • Unicast IP routing

  • IP multicasting

  • NAT

Demand-Dial Routing

Demand-dial routing (also known as dial-on-demand routing) is the forwarding of packets over Point-to-Point Protocol (PPP) links within a wide-area network (WAN) infrastructure of dial-up telecommunications technologies, including Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), X.25, and Asynchronous Transfer Mode (ATM) over asymmetric digital subscriber line (ADSL). Demand-dial routing allows you to connect branch offices through the Internet and to implement dial-up site-to-site virtual private network (VPN) connections.

Because demand-dial routing is an implementation of Routing and Remote Access technologies, including unicast routing and multicasting, it is integrated within the Routing and Remote Access architecture. For information about the architecture of Routing and Remote Access, see Unicast IPv4 Routing Technical Reference and IPv4 Multicasting Technical Reference.

Unicast IP Routing

Unicast routing forwards packets from one host to another host using the unicast destination IP address.

The Unicast IP Routing subject describes Internet Protocol version 4 (IPv4) unicast routing technologies included in the Routing and Remote Access service. The IP networking protocol is fundamental to the unicast routing that occurs on any Windows-based IPv4 network. This includes networks that use hardware routers, software-based routers, or a combination of both.

In addition to supporting IP routing, Windows Server 2003 also supports Appletalk routing; however, Windows Server 2003 does not support Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) routing. Although, at some time in the future, IPv4 might be replaced by the next generation of IP, IP version 6 (IPv6), at present most unicast IP routing takes place over IPv4 networks. Windows Server 2003 supports both IPv4 and IPv6. This subject describes unicast IP routing in an IPv4 environment. For more information about Windows Server 2003 and IPv6, see “IPv6 Technical Reference.”

IP Multicasting

Internet Protocol (IP) multicasting is the delivery of a single IP datagram to a group of hosts on a TCP/IP-based network. IP multicasting is the most practical method for point-to-multipoint data transmission because unicast IP routing requires multiple copies of the datagrams, and broadcast does not work across routers.

The IP Multicasting subject describes the IPv4 multicast forwarding functionalities included in the TCP/IP (IPv4) protocol and the Routing and Remote access service.


Windows Server 2003 provides network address translation (NAT) functionality as part of the Routing and Remote Access service. NAT provides a method for translating the IPv4 addresses of computers on one network into IPv4 addresses of computers on a different network. A NAT-enabled IP router deployed at the boundary where a private network meets a public network allows computers on the private network to access computers on the public network by providing this translation service.

Routing Scenarios

The ability for people to communicate by sending messages from one computer to another, whether the computers are located on the same network or on networks at opposite sides of the globe, has become commonplace. These communications are supported by routing methods described in the following scenarios.

Demand-dial routing scenario

Organizations looking to reduce the telephone costs incurred by their dial-in users may want to consider implementing demand-dial routing. Demand-dial routing reduces dial-in costs because the connection becomes active only when data is sent to the remote site. When no data has been sent over the link for a specified amount of time, the link is disconnected. By making a demand-dial connection, you can use existing dial-up telephone lines instead of leased lines for low-traffic situations. Demand-dial routing also provides organizations the ability to specify the hours that a router is allowed to dial out to make demand-dial connections. Demand-dial routing can significantly reduce connection costs.

Unicast routing scenario

Unicast routing over IP internetworks is a major part of the technology that makes such communication possible.

Today, the majority of internetwork traffic worldwide is over IPv4 networks, and most user-initiated traffic across IPv4 internetworks is unicast traffic. Any Windows-based network supports unicast IP routing, including networks that use only hardware routers, networks that use software-based routers such as that provided by the Windows Server 2003 Routing and Remote Access service, or a combination of hardware and software routers.

IP multicasting scenario

Recent trends in multimedia transmission across intranets and the Internet, such as video conferencing, live event broadcasting, and distance learning, require a bandwidth-conserving method of delivering data to multiple interested hosts. Of the delivery methods supported by IP, multicasting is the method that best addresses this need.

NAT scenario

The impetus initiating the development of NAT technology was the need to provide a temporary solution to the IPv4 address-depletion problem faced by the Internet community. The number of available globally unique (public) IPv4 addresses is far too few to accommodate the rapidly increasing number of computers that need access to the Internet. Although the long-term solution — the development of Internet Protocol version 6 (IPv6) addresses — exists, IPv6 is not yet widely adopted. NAT technology lets computers on any network use reusable private addresses to connect to computers with globally unique public addresses on the Internet.

NAT enables computers on small- to medium-sized organizations with private networks to access resources on the Internet or other public network. The computers on a private network are configured with reusable private IPv4 addresses; the computers on a public network are configured with globally unique IPv4 (or rarely, at present, IPv6) addresses. A typical deployment is a small office or home office (SOHO) or a medium-sized business that uses Routing and Remote Access NAT technology to enable computers on the internal corporate network to connect to resources on the Internet without having to deploy a proxy server.