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Classful Addressing and Subnetting

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Subnet Masks ... Subnet Masks /24. 11111111.11111111.11111111. ... Are Subnet Masks Redundant? Why bother, if only class A, B and C networks are allowed? ... – PowerPoint PPT presentation

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Title: Classful Addressing and Subnetting


1
Classful AddressingandSubnetting
  • Richard Gnall

2
Class A Network
  • First bit of first octet is set equal to 0

Host Address
Host Address
24 bits
24 bits
00000000 - 01111111.X.Y.Z
0 - 127.X.Y.Z
7 free bits
7 free bits
Network Address
Network Address
3
Class B Network
  • First two bits of first octet is set equal to 10

Host Address
Host Address
16 bits
16 bits
10000000 - 10111111.X.Y.Z
128 - 191.X.Y.Z
14 free bits
14 free bits
Network Address
Network Address
4
Class C Network
  • First three bits of first octet is set equal to
    110

Host Address
Host Address
8 bits
8 bits
11000000 - 11011111.X.Y.Z
192 - 223.X.Y.Z
21 free bits
21 free bits
Network Address
Network Address
5
Class D Network
  • First four bits of first octet is set equal to
    1110

Host Address
Host Address
8 bits
8 bits
11100000 - 11101111.X.Y.Z
224 - 239.X.Y.Z
20 free bits
20 free bits
Network Address
Network Address
6
Class E Network
  • First four bits of first octet is set equal to
    1111

Host Address
Host Address
8 bits
8 bits
11110000 - 11111111.X.Y.Z
240 - 255.X.Y.Z
20 free bits
20 free bits
Network Address
Network Address
7
Network Classes
8
Reserved Address Ranges
  • 10.X.Y.Z Private Intranet
  • 172.16-32.Y.Z Private Intranet
  • 192.168.Y.Z Private Intranet
  • 169.254.Y.Z APIPA
  • 224 - 239.X.Y.Z Class D Multicasting
  • 240 - 255.X.Y.Z Class E Research

Non-Routable on the Public Internet
9
Restricted Addresses
  • Network ID/Number
  • First IP address in a subnet range
  • All host bits set to 0
  • Default Gateway (Router) Address
  • Usually, but not necessarily, the second IP
    address in a subnet range
  • Specifies where to send packets destined for a
    different subnet
  • If the subnet is not connected to a router, this
    IP address does not need to be reserved for a
    gateway
  • Broadcast Address
  • Last IP address in a subnet range
  • All host bits set to 1
  • Packet will be read by all hosts
  • Loopback Address
  • 127.X.Y.Z
  • 127.0.0.1 ? implies no 127.X.Y.Z
  • Messages don't go out on the network, but instead
    just test the installed IP software
  • All pings sent to any 127.X.Y.X address get sent
    to 127.0.0.1

10
Subnet Masks
  • Additional piece of information identifying the
    network and host sections of the IP address
  • A binary 1 for every bit that belongs to the
    network portion of the address.
  • A binary 0 for every bit that belongs to the host
    portion of the address.
  • IP Address 10110110.11011011.01011010.00110011
  • Subnet Mask 11111111.11111111.00000000.00000000
  • Subnet Mask 255.255.0.0
  • CIDR Notation /16
  • Network ID 10110110.11011011.00000000.00000000

11
Subnet Masks
12
Are Subnet Masks Redundant?
  • Why bother, if only class A, B and C networks are
    allowed? Isn't it obvious which part of the IP
    address is the network address (by examing the
    first one to four bits of the IP address)? There
    should just be three choices
  • 255.0.0.0, 255.255.0.0, or 255.255.255.0
  • A network can be broken up into smaller networks
    (subnets) by borrowing some of the host bits to
    help denote a network address. Instead of using
    all or none of the bits in a single octet to
    define the network, a subset of the bits can be
    used to define the network.
  • Example
  • To subdivide a Class A network into two subnets,
    use the mask
  • 11111111.10000000.00000000.00000000 255.128.0.0 /9
  • To subdivide a Class A network into four subnets,
    use the mask
  • 11111111.11000000.00000000.00000000 255.192.0.0 /1
    0

13
Are Subnet Masks Redundant? (cont.)
  • To subdivide a Class A network into two subnets,
    use the mask
  • 11111111.10000000.00000000.00000000 255.128.0.0 /9
  • If the class A network is 50.X.Y.Z, then the two
    subnets would be
  • 50.0-------.Y.Z
  • 50.1-------.Y.Z
  • To subdivide a Class A network into four subnets,
    use the mask
  • 11111111.11000000.00000000.00000000 255.192.0.0 /1
    0
  • If the class A network is 50.X.Y.Z, then the four
    subnets would be
  • 50.00------.Y.Z
  • 50.01------.Y.Z
  • 50.10------.Y.Z
  • 50.11------.Y.Z

14
RFC Restricted Subnet Values
  • Prior to 1995, RFC 950 ("Internet Standard
    Subnetting Procedure") disallowed the use of all
    0's or all 1's within a single octet to denote
    subnets. It was felt that some routers would be
    confused by this type of subnetting.
  • So, using a single bit to subnet a Class A, B, or
    C network is pointless as the first subnet is all
    0's (a single 0) and the second subnet is all 1's
    (a single 1).
  • Using 2 bits to subnet a network, results in two
    useful subnets, 01 and 10. The first and the last
    are disallowed, 00 and 11.
  • In general, prior to 1995 and under the rules of
    RFC 950, n bits gives 2n - 2 usable subnets.
    (This is important for CISCO certification
    exams.)
  • In 1995, RFC 1812, titled "Requirements for IP
    Version 4 Routers", lifted this restriction.
  • In general, after 1995 and under the rules of RFC
    1812 , n bits gives 2n usable subnets.
  • Note however, that both Microsoft and CISCO
    TCP/IP certification exams follow the earlier RFC
    950 convention.

15
RFC Restricted Subnet Values
  • RFC 950/CCNA Subnet Restrictions
  • The first and last subnets in a subnetted network
    can not be used. So if a network has been
    subdivided into two subnets, there are in effect
    no available subnets. If a network has been
    subdivided into four subnets, there are in effect
    only two available subnets.
  • In each octet, the bits in the portion of the
    network address which represent the network ID
    can not be all 0's or 1's. This slightly reduces
    the number of available subnets.
  • ____.____.____.00-------- X not allowed
  • ____.____.____.01-------- allowed
  • ____.____.____.10-------- allowed
  • ____.____.____.11-------- X not allowed
  • Technically speaking, the first and last subnets
    would work fine with modern routers, but for
    Microsoft and Cisco certification exams, the
    first and last subnets are not allowed.
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