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what is YY in X.X.X.X / YY

By thinknologist ·
To all,

Please explain what is YY in the IP address format X.X.X.X/YY . I am really confused with this numeric value. Kindly raise a brief explanation and sample scenario for me to adapt this. A site explaining this would be a big help


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subnet mask

by JOE.HUTCHINSON1 In reply to what is YY in X.X.X.X / Y ...

x.x.x.x/8 would be x.x.x.x
x.x.x.x/16 would be x.x.x.x
x.x.x.x/24 would be x.x.x.x
x.x.x.x/32 would be x.x.x.x

this is just an easier way of putting the sub netmask in short hand. save putting it in full
its just the binary digits put in 1 number. sorry if this does not make it clear never was good at explaining things

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Classless Inter-Domain Routing (CIDR)

by SamZa In reply to what is YY in X.X.X.X / Y ...

As Mr. Hutchinson said in his reply, the YY in the notation simply indicates the subnet mask using a notation called CIDR, or Classless Inter-Domain Routing. Because Class A, B, and C addresses were becoming unwieldy and wasteful (very few companies truly need all of the host addresses available in a class B address), CIDR was developed to allow a company to buy only those addresses it truly needed. Thus, if I needed 4,000 addresses, I would have a CIDR address of /20, giving me, for example, a possible IP address of for one of my addresses. The /20 simply means I used 20 bits in my subnet mask, so I would have a binary subnet mask of 11111111.11111111.11110000.00000000, which would translate back to a subnet mask in base 10 (decimal) of This subnet mask would give me 14 class B networks with a possible number of hosts on each network of 4094. The possible number of hosts is derived by taking the number of zeros left over in my subnet mask (in this case, you can count 12 zeros left over, and applying the formula ((2 to the 12th power)-2). 2 to the 12th is 4,096, -2 is 4094, so I could have 14 networks with 4,094 hosts per network (the -2 comes from using Microsoft's assumption that you can't use the first and last range in a range of host addresses). This is a real quickie explanation, but I hope it helps.

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Could also be TCP port

by rickpop In reply to what is YY in X.X.X.X / Y ...

Depending where you are trying to enter this data, it could also be a TCP port number. In cisco v12x OS, this syntax is also used to desiginate a specific TCP port associated with an IP address. i.e.;

ip nat inside source static tcp 80 80 extendable
ip nat inside source static tcp 25 25 extendable

The first example is for regular "http" traffic on TCP port 80.
The second is for "SMTP email" traffic on TCP port 25.

For a list of common TCP ports, you could look at:

Hope this helps

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I think you missed the / (slash)

by kiddanger In reply to Could also be TCP port

CIDR is what is being referenced here.

IP addresses with ports are generally written as:

Ex. will get you to Microsoft's web site via HTTP but: will get you to their custom 404 error page.

Also, you're referencing IOS and using spaces as a delimiter, not a slash.

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by ahmed.meki In reply to what is YY in X.X.X.X / Y ...

the ip address format X.X.X.X/YY is called CIDR network address it means this ip X.X.X.X contains YY bits for newtwork address and 24-YY bits for hosts addresses

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to simplify

by lcampbell In reply to

The x.x.x.x is the IP address. The yy is the number of bits that are turned on to create the subnet mask.
more info at

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