Subnetting

Hi, I hope I posted this in the right section.

I am trying to figure it out what is the calculation needed to determine the subnet mask for a host.I know what class is the ip and what default subnet mask it's got.

I have read something about some borrowing thing,from the last octect of the ip address(the host) but I cannot understand it. I guess it is related to the prefix (i.e /24)


Please, help me...I am going nuts with this
5 answers Last reply
More about subnetting
  1. That is not much help as I ve been using Google all the day,compared a few pages. What I am saying is that I don't get it.

    Could someone help?
  2. damian86 said:
    Hi, I hope I posted this in the right section.

    I am trying to figure it out what is the calculation needed to determine the subnet mask for a host.I know what class is the ip and what default subnet mask it's got.

    I have read something about some borrowing thing,from the last octect of the ip address(the host) but I cannot understand it. I guess it is related to the prefix (i.e /24)


    Please, help me...I am going nuts with this



    To determine the subnet mask you need to ask yourself how many host addresses you need for the subnetwork. Say you need room for 75 hosts on the network, you'll need 7 host bits (2^7-2 addresses for the network and broadcast addresses) which will give you enough room for 126 hosts. 6 host bits will provide you only enough room for 62 hosts so that's no good; 8 bits in the host portion of the address will give you enough room for 254 hosts (too many if your trying to be efficient with your address space). The subnet mask for the 75 host network would be 255.255.255.128; in binary 11111111.11111111.11111111.10000000 (the zeros are the 7 host bits, 1's are network bits). For example, if your subnetting the 192.168.1.0 network using fixed length subnet masks:

    First Sub-Network
    Network address. 192.168.1.0 /25
    First Host: 192.168.1.1
    Last Host: 192.168.126
    Broadcast: 192.168.1.127

    Second Sub-Network
    Network address. 192.168.1.128 /25
    First Host: 192.168.1.129
    Last Host: 192.168.254
    Broadcast: 192.168.1.255

    Watch this series of videos: http://www.youtube.com/watch?v=XFB33GKQdOs. Ill happily answer any questions you have.

    I also have access to the cisco 1 & 2 course curriculum if you'd like. Also, here is the link to packet tracer: http://www.packettracer.info/. It'll allow you to build virtual networks you can play around with, without the cost and pressure of working with live physical equipment.
  3. Thanks Jsmith,

    I think this is what I needed to read, the videos are very good I am starting to understand it, I would watch it again to practice. The thing is that I am doing Cisco as well and it is really difficult for me and the others as I see.

    I will be playing with the packet tracer later on once I get it well.

    The Cisco material they are giving us right now is too full of words, too complex and in my opinion it doesn't come very well explained, I always have to come home after college and use the internet to find help which will take me less time than 'trying' to read the Cisco chapter, I probably read it and don't get anything.

    I am writing you this in advance just to let you know that it has been helpful and that I am on it. Thank you very much. :)
  4. Can you help me with the IP prefix thing?
    I got this on my Cisco curriculum but I cannot understand it

    The prefix length is the number of bits in the address that gives us the network portion. For example, in 172.16.4.0 /24, the /24 is the prefix length
Ask a new question

Read More

Subnet Networking