Problem: What are valid IP Addresses for a private network?

Solution: RFC, (Request For Comments), 1918, Address Allocation for Private Internets, recommends IP address ranges one can use for hosts that do not require direct access to the Internet. These addresses are filtered by Internet Routers and therefore do not have to be globally unique. 

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private networks:

10.0.0.0 - 10.255.255.255

172.16.0.0 - 172.31.255.255

192.168.0.0 - 192.168.255.255

Note:  The first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and the third block is a set of 255 contiguous class C network numbers. The complete RFC 1918 can be found via FTP on nic.ddn.mil.

Obtaining RFCs

RFCs may be obtained by using e-mail or the File Transfer Protocol (FTP) from many RFC repositories. Primary Repositories should have the RFC available when it is first announced. Secondary Repositories may have the RFC available when it is first announced or they may need a few days to obtain the most recent RFCs. 

Many of these repositories now have World Wide Web servers. http://www.isi.edu/rfc-editor/ has more information about RFCs.

RFCs can be obtained by FTP from any of the following:

nis.nsf.net
nisc.jvnc.net
ftp.isi.edu
wuarchive.wustl.edu
src.doc.ic.ac.uk
ftp.ncren.net
ftp.sesqui.net
ftp.nic.it
ftp.imag.fr


To obtain RFCs from nis.nsf.net by using FTP, use the following steps:

1.Login with username "anonymous" and password "name@host.domain".

2.Type "cd /internet/documents/rfc" (without quotation marks), and then press ENTER. Files that are stored in this folder use the following naming convention

rfc<nnnn>.txt

where <nnnn> is the RFC number.

NOTE: For sites without FTP capability, you can send e-mail queries to nis.nsf.net. Address the request to "nis-info@nis.nsf.net" (without quotation marks), but leave the subject field of the message blank. The first line of text in the message must be "send rfc<nnnn>.txt" (without quotation marks), where <nnnn> is the RFC number.