CCNA - Help Understanding Example: 131.107.2.56/28

Author

Help Understanding Example: 131.107.2.56/28

Ybentrepreneur

2003-01-27, 9:45 pm

I hate to say it, but I really dont understand the /24, /27 etc on Ip Addresses. I know this must be very simple. I understand how to subnet networks such as 120.0.0.0, 192.168.5.0 networks very well, I just dont understand that / .

How do I see on a router what subnet I am using? When I type, Sh run on my little internetwork design, I get something like 120.0.0.4/12. How exactly does this work in detail????

Also, after I figure this out, and perfect it, i should be ready for this exam. How much does this exam cost????

on the exam, can i use short commands such as E0, SH, INT s0, Encap??

Thanks a lot fellas.

davidbeecken

2003-01-27, 11:09 pm

all the /xx is, is the number of masked bits in the subnetmask. IE /8 is 255.0.0.0

loopbacklady

2003-01-28, 4:52 am

Your example is a class B address, so 16 of those /28 bits are the network portion. That leaves 12 bits that were used for subnetting-all of the 3rd octet and 4 bits from the 4th octet.

mikop

2003-01-28, 5:13 am

quote:
Originally posted by loopbacklady
Your example is a class B address, so 16 of those /28 bits are the network portion. That leaves 12 bits that were used for subnetting-all of the 3rd octet and 4 bits from the 4th octet.

no,

/bit is just that, it tell you how many bits you are using for your network portion.

so his example is simply... 28 bits are used to indicate the network.

class A B C does not factor in.

so it is NOT 16 bit of those 28 bits are network and the remaining are use for subnet. it.

so

131.107.2.56/28 mean

a network of
131.107.2.48
broadcast
131.107.2.63
usable range
131.107.2.49-62

/28 mean all 28 bits are the network ID, not 16 bits since it is *class B*

The very concept of class, altho important for learning purposes, is outdated and non existent.

edit:

http://www.3com.com/other/pdfs/infr...n_US/501302.pdf

everyone should read that from top to bottom. It is informative in providing background so you understand the challenges and resolution. It has ample examples to walk you through and also examples at the end of the file. It is quite long. (50 pages or so, but this is great investment of your time) Yshould and need to know this and it will benefit you greatly in later studies in the network field, both network equip and network OS.

Sexy Lexy

2003-01-28, 1:08 pm

Mikop is correct and has given an excellent explanation.

The / followed by a number indicates the number of bits in the subnet mask. This is called Classless InterDomain Routing as the class of the address is irrespective.

In this example the /28 indicated that the 28 bits are used in the subnet mask (Right to left precedence) this indicated the network portion of the address, remember it is classless.

This table should give you some indication:

Cisco Hosts and Subnet Quantities

Hope this helps.

djmaplethorpe

2003-01-28, 4:18 pm

quote:
Originally posted by Sexy Lexy
Mikop is correct and has given an excellent explanation.

The / followed by a number indicates the number of bits in the subnet mask. This is called Classless InterDomain Routing as the class of the address is irrespective.

In this example the /28 indicated that the 28 bits are used in the subnet mask (Right to left precedence) this indicated the network portion of the address, remember it is classless.

This table should give you some indication:

Cisco Hosts and Subnet Quantities

Hope this helps.

Great link

thanks

loopbacklady

2003-01-28, 5:14 pm

ok, sorry. I was just trying to explain it simply, with the example given, the way I learned it myself.

e_mc2

2003-01-28, 5:15 pm

make it simple enough in case you still dont understand.

ex. 192.168.30.2/28 or /8 or /16 or whatever.
write it out in binary and you will see it little clearer.

for /28 means

11111111.11111111.11111111.11110000

(by looking at the Ip address...of 192...you know this is a class C address)

again... for /8 mean

11111111.00000000.0000000.00000000

hope this help...

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