Reverse Address Lookup

E-Mail service and Reverse Address Lookup

One of the components of the Internet is DNS - domain name service. In part, DNS allows for the matching of IP addresses and system names. Each ISP - Internet Service Provider - is responsible for updating and managing their DNS tables. These DNS files are shared with other ISPs so that each address and system is recorded.

To use the RPI campus e-mail service - mail.rpi.edu - a successful reverse DNS lookup is required. Reverse DNS lookups are an important component of our anti-spam implementation. Other (but not all) services and service providers on the Internet also require DNS lookups.

If you are using a non-RPI ISP such as RoadRunner and they do not keep their DNS tables up-to-date you will not be able to access the mail.rpi.edu server. The RPI mail server refuses connections from any system that does not have a valid reverse map.

From time to time an ISP will expand the IP (Internet Protocol) range that they use. Occasionally, these new IP addresses are not added (registered) to the DNS tables. If your connection through this ISP gets a non-registered IP address, connections to mail.rpi.edu will be refused.

How will you know? It is real hard to know without doing a check. You can check the status of your IP address by logging onto one of the remote access IBM systems: rcs-ibm1.rpi.edu or rcs-ibm2.rpi.edu and running the nslookup utility as shown below.

Checking for a Valid IP Address

You will need to know your system's IP address. On your Windows system, do the following:

Open a command window: from the Start menu click Run and in the Run box enter command
Once the command window opens:
- enter winipcfg (Window 98)
  Be sure to find the IP address of your network adapter card and not the PPP adapter.
- enter ipconfig (Windows 2000 and newer)
After you have logged into an IBM remote access server use the IP address as follows (substituting your IP address for the 128.113.173.118 listed below):
A successful lookup would look like:

nslookup 128.113.173.118
Server: netserv6.its.rpi.edu
Address: 128.113.92.10
Name: tejanb-2.stu.rpi.edu
Address: 128.113.173.118

An unsuccessful lookup will look like:

nslookup 128.113.15.20
Server: netserv6.its.rpi.edu
Address: 128.113.92.10
*** netserv6.its.rpi.edu can't find 128.113.15.20:Non-existent host/domain

If the lookup fails you will need to contact your ISP. If it does not fail contact the VCC Help Desk for help.

Last modified: July 26, 2004