Data Comm for Business, Inc. LAN WAN PPP SR Simple Router

Data Comm for Business, Inc.

Your Complete Data Communications Provider Since 1981

Conserving IP Addresses with the PPP-SR Simple Router

Abstract:

Internet Service Provides and corporations interested in providing PPP connections to a single PC or small groups of PCs are increasingly turning to the DCB PPP-SR Simple Router product line for simplicity and economy. This product connects a remote workstation (usually a Windows 95 PC) to a host TCP/IP network without using a local LAN, NIC cards, or hub. It communicates with the host network using PPP WAN ports or a common Frame Relay port. IP addresses are in short supply, so it is imperative that routers and workstations be configured to minimize the use of IP address space. This paper describes a method of configuring a host router and the PPP-SR products to meet that goal by allowing up to 128 remote sites to exist in a single class C address space.

Details:

Examining RFC 1860, RFC 950, STD 3 , and STD5, and especially the tables in RFC1878 show that a class C IP address space can be subnetted to 128 subnets of 2 hosts each (using the current practice of allowing a zero node address, or 64 subnets of 2 hosts each using the obsolete practice of disallowing zero node addresses ). When using PPP-SR01 or PPP-SRDSU-01 simple routers to connect a single remote node (usually a Windows 95 machine), this subnetting on the host router provides two IP addresses for each connection... one for the router WAN port and one for the remote node machine. The host router and remote node machine should be configured with a network mask of 255.255.255.252 . Note that other routers (and different routes on the host router) CAN use a different network mask to correspond with the overall network subnetting scheme. An example of this method is shown in figure 1.



Example Configuration:
Host Router
Port
Host Port
IP Address
Host Port
Mask
Remote Site
IP Address
Remote Site
Mask
1 192.16.4.1 255.255.255.252 192.16.4.2 255.255.255.252
2 192.16.4.5 255.255.255.252 192.16.4.6 255.255.255.252
3 192.16.4.9 255.255.255.252 192.16.4.10 255.255.255.252
4 192.16.4.13 255.255.255.252 192.16.4.14 255.255.255.252
...

If using the PPP-SR04 or PPP-SRDSU04 (which allows four computers to be connected to a single dedicated line without local ethernet networking by utilizing the Windows Dial-Up-Networking wizard and the COM: port), then there should be enough node numbers allocated for up to 6 nodes on the subnet. This allows four addresses for workstations, one for the PPP-SR, and one for the host router port. The PPP-SR does not NEED an IP address, but it is useful for troubleshooting the system. The network mask for the host router should be 255.255.255.248. This mask provides for 32 subnets in a class C space. This works for both the obsolete non-zero node number and the current zero node number allocations because both support 6 nodes with this mask. The example in figure 2 presumes four workstations at each remote site.


Example Configuration:
Host Router
Port
Host Port
IP Address
Host Port
Mask
Remote Site
Port
Remote Site
IP Address
Remote Site
Mask
1 192.16.4.1 255.255.255.248 1 192.16.4.2 255.255.255.248
2 192.16.4.3 255.255.255.248
3 192.16.4.4 255.255.255.248
4 192.16.4.5 255.255.255.248
2192.16.4.9255.255.255.248 1192.16.4.10 255.255.255.248
2 192.16.4.11 255.255.255.248
3 192.16.4.12 255.255.255.248
4 192.16.4.13 255.255.255.248 ...

The addressing schemes listed above will work with virtually any router using either Sync PPP or Frame Relay. Some routers (usually working with Frame Relay transport) can be configured without an IP address on their WAN ports. These routers can use an even finer grained subnet mask (255.255.255.255) which allows 254 remote sites (each with one PC) in a single class C address space.

Note that the PPP-SR products do not require an IP address. However, if one is configured, it can be used to ping (either to or from) the PPP-SR. This is useful when first getting the system up and for future troubleshooting or link verification. The remote computers are always configured to use DUN, and there is usually no networking expertise available at the remote site to assist in troubleshooting. Being able to ping the PPP-SR and the remote computer allows connectivity problem diagnosis to the lowest level without remote networking expertise.



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