Subnet Question

Discussion in 'General Mac Discussion' started by seabass069, May 15, 2006.

  1. seabass069 macrumors regular

    Joined:
    Oct 5, 2005
    #1
    Is there a good website that explains CLASS C SUBNET in basic terms?
     
  2. Queso macrumors G4

    Joined:
    Mar 4, 2006
    #2
    Difficult to explain without going into detail and a lot of people get this confused. Firstly, a network is only a subnet when it is using more bits in the subnet mask than the natural mask for it's class. The IP address clesses are defined by the first three bits of the IP address when written in binary.

    Addresses beginning 1-127 are Class A (first octet starts 010 in binary). Class A networks naturally have 8 bit masks (255.0.0.0)
    Addresses beginning 128-191 are Class B (irst octet starts 100 in binary) Class B networks naturally have 16 bit masks (255.255.0.0)
    Addresses beginning 192-223 are Class C (irst octet starts 110 in binary) Class C networks naturally have 24 bit masks (255.255.255.0)

    The remaining addresses between 224 and 255 are used for either multicasting or for experimental reasons.

    For example;

    A Class C network is therefore always in the range 192-223 in the first octect of the IP address, and always has a 24-bit subnet mask. A Class C subnet will be a sub-division of a Class C network, for example 195.1.3.64 with a subnet mask of 255.255.255.192.

    Here's where people get confused. 10.42.5.0 with a 24-bit subnet mask isn't a Class C subnet. It's a Class A subnet, as it naturally belongs in the network 10.0.0.0/8, but that network has been divided by manipulating bits in the subnet mask. You can also get Supernets where bits are taken from the natural masks to increase the available number of addresses available to hosts. For example 192.168.128.0 with a 23 bit mask is a Class C supernet (it allows 510 hosts rather than the normal Class C 254)

    I think your best bet for a definition is to look up Classless Inter Domain Routing (CIDR). That explains how subnet masks can be manipluated to divide an address space.

    Here's one here:-

    http://www.petri.co.il/what's_cidr.htm
     

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