Simple Question.

I will be getting into CCNA and cisco Later but I am currently reading the book Extreme Exploits By Victor Oppleman, Oliver Fried richs and Brett Watson. And whenever they give an Address Prefix, which i don't yet understand: its in this form.

for example:

192.168.0.0/16

What does the /16 mean, something about an autonomous system number or something? Im kinda confused.[/i]

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optimus

Well, I know that 192.168.0.0 is a class C address which means that the first three octets must be used for the network address. This would be the 192.168.0 part, which then must also use ip subnet zero.

What 192.168.0.0/16 is telling me is that only the first two octets ( 8 bits + 8 bits ) are used for the network address. Since a class C must use the first three octets, which would constitute 192.168.0.0/24 , this just does not make any sense to me.

- Optimus

xwesleyxwillisx

192.168.0.0 /16 is one of three private reserved IP subnets. 10.0.0.0 /8 and 172.16.0.0 /12 being the others. The /16 notation is called CIDR or Classless Interdomain Routing. It simply tells you how many bits are used to identify the network. These subnets are special because they are used for private LAN's. Basically, every LAN can use these subnets for their internal addresses. However, for WAN connectivity public IP addresses (anything not on those 3 subnets) must be leased through an ISP. All public IP addresses are registered with ARIN (American Registry for Internet Numbers) for organization and control. Hope this helps!

1Ste

xwesleyxwillisx wrote:

192.168.0.0 /16 is one of three private reserved IP subnets. 10.0.0.0 /8 and 172.16.0.0 /12 being the others. The /16 notation is called CIDR or Classless Interdomain Routing. It simply tells you how many bits are used to identify the network. These subnets are special because they are used for private LAN's. Basically, every LAN can use these subnets for their internal addresses. However, for WAN connectivity public IP addresses (anything not on those 3 subnets) must be leased through an ISP. All public IP addresses are registered with ARIN (American Registry for Internet Numbers) for organization and control. Hope this helps!

Thanks, makes since now.

texican

In the general case (in addition to the three private address spaces mentioned), the /xx is just another way of putting the subnet mask without having to type out the four octets in addition to the IP address. The /xx stands for the number of bits turned on in the mask, so a /16 mask equates to 11111111.11111111.00000000.00000000 (first 16 bits set to "1"). /24 is pretty common too, but you'll see bigger numbers such as /30 (255.255.255.252) when it comes to WAN links, for example, to conserve public IP addresses as mention by xwesleyxwillisx. If you need to know what the four-octect version of the mask is, just write out the bits and do the binary-to-decimal conversion.

mikeyoung

Excellent and accurate explanaitons, but here is another one that helped me: the /xx is used to show how many "1"s there are in the subnet mask (yes it has a similar meaning as "the number of bits that are used").

/16 = 16 1s = 11111111.11111111.00000000.00000000 = 255.255.0.0

Mike

You might want to check this link that gives an idea about supernetting:
http://www.techexams.net/forums/viewtopic.php?t=11417

cisco_trouble

CIDR which stands for Classless Inter-domain Routing is a new addressing scheme for the Internet which allows for more efficient allocation of IP addresses than the old Class A, B, and C address scheme.
It basically means increasing the amount addresses that are currently available.