[Dshield] Another simple question...

John Sage jsage at finchhaven.com
Sun Oct 28 18:08:15 GMT 2001


/8 /12 /24 /32 are what is known as CIDR (Classless InterDomain Routing) 

These refer to the size, in bits, of the mask that separates the network 
portion of an IP address from the host portion.

Basically this allows segregation of the IPV4 address space at will by 
single bits in the netmask, rather than the older method of using only 
class A, class B, class C etc networks which were defined by entire 

/8 means that that first eight bits -- 10. in the example below, or 212. 
for another example -- is the network portion.

/12 indictates that the first twelve bits -- 172.16. -- are the network 
portion; etc..

See: ftp://ftp.isi.edu/in-notes/rfc1878.txt for enough detail to satisfy 
most anyone; also see "TCP/IP Illustrated vol. 1" W.R. Stevens, pp. 140-141

A handy tool for messing about with this stuff can be found at: 
http://jodies.de/ipcalc -- which you can either use up there, or 
download, install, and run locally...


- John

Josh Tolley wrote:

> What does the /8, /12, etc. below mean?  I assume subnet, but having
> only used 4 octet notation for subnets, I don't know how to translate
> it. 
> Josh Tolley
> <snip>
> The reserved-for-private-use netblocks should be blocked in *both 
> directions* (in and out, source and destination) at your boundary: 
> </snip>

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