Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Finding a Subnet Mask

Any one have an idea that how can we find a subnet mask from an IP range ?

Example:

If we have a range of IP addresses Like

1.1.0.1 --> 1.1.5.254

Please help

3 REPLIES

Re: Finding a Subnet Mask

See http://www.vlsm-calc.net/

Sent from Cisco Technical Support iPad App

Cisco Employee

Re: Finding a Subnet Mask

1.1.0.1 -> 1.1.5.254

Let's translate both addresses into binary form:

00000001.00000001.00000000.00000001 -> 00000001.00000001.00000101.11111110

Let's see which bit's have changed from the lowest to the highest address:

00000001.00000001.00000[000.00000001 -> 101.11111110]

So we can see that  21 bit left the same, and 11 bits changed:

If we really picked up the first and the last addresses we can assume that  network portion (or network mask) is 21 bits, and host portion is 11 bits.

So your network address is 1.1.0.0 (because network address is where all host bits are set to 0 )

And your broadcast address is 1.1.5.255 (all bits set to 1)

Network can be written ad 1.1.0.0/21 or 1.1.0.0 255.255.248.0

HTH,

Alex

Cisco Employee

Re: Finding a Subnet Mask

Also you can become expert in subnetting by playing the 'subnet game' . Just visit the link below-

https://learningnetwork.cisco.com/docs/DOC-1802

701
Views
20
Helpful
3
Replies