Question on IPv4CompTIA Network+ Course

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This topic contains 2 replies, has 3 voices, and was last updated by  cliffwilliams 2 years, 2 months ago.

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    Can anyone explain me how Class A IP addressed have 128 network as Anthony said on his lectures? 0-126 is the range, so how 128?



    May have been a mistake. Class A has 126 networks: 1-126

    “The high-order bit in a class A address is always set to zero. The next seven bits (completing the first octet) complete the network ID. The remaining 24 bits (the last three octets) represent the host ID. This allows for 126 networks and 16,777,214 hosts per network. ”



    I don’t think it was a mistake.

    You have to remember that classful addressing is old. It was used between 1981 and 1993 but replaced by classless inter-domain routing in 1993. Despite this the concept is still widely used by network engineers as a sort of shorthand. In doing so they often mix it up with later classless concepts such as subnet masks. In fact under classful addressing you did not need a subnet mask.

    Under classful addressing, the class of address is determined by the first few bits of the address. If the first bit is 0 then it is class A and the next 7 bits are used for the network address. With 7 bits you can have a total of 128 network addresses. From this point of few what Anthony says is correct. However you also have to remember that certain network addresses are reserved for other purposes. Both the network addresses of 0.x.x.x & 127.x.x.x are reserved for special purposes. Leaving 126 other addresses available for normal use. In practice therefore people often talk about there being 126 usable network addresses in class A. Hope this makes some sense

    The classful addressing was part of RFC 791 which in the section on addressing states the following

    Addresses are fixed length of four octets (32 bits). An address
    begins with a network number, followed by local address (called the
    “rest” field). There are three formats or classes of internet
    addresses: in class a, the high order bit is zero, the next 7 bits
    are the network, and the last 24 bits are the local address; in
    class b, the high order two bits are one-zero, the next 14 bits are
    the network and the last 16 bits are the local address; in class c,
    the high order three bits are one-one-zero, the next 21 bits are the
    network and the last 8 bits are the local address.

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