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Subnetting the IPv4 Address Space

Subnetting the IPv4 Address Space. BSAD 141 Dave Novak Sources : Network + Guide to Networks, Dean 2013. Outline. IPv4 address space review Subnetting conceptual review Subnetting example. Classes of IP addresses. Classes A, B, and C are primary classes Used for host assignments

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Subnetting the IPv4 Address Space

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  1. Subnetting the IPv4 Address Space BSAD 141 Dave Novak Sources: Network+ Guide to Networks, Dean 2013

  2. Outline • IPv4 address space review • Subnetting conceptual review • Subnetting example

  3. Classes of IP addresses • Classes A, B, and C are primary classes • Used for host assignments • Class D is used for multicast • The class is determined by boundary between the network prefix and the suffix

  4. Classes and dotted decimal Any value in range ALWAYS will have “0” as first bit Any value in range ALWAYS will have “10” as first two bits Any value in range ALWAYS will have “110” as first three bits Example: Class A – 119.x.x.x = 01110111 Example: Class B – 190.x.x.x = 10111110 Example: Class C – 200.x.x.x = 11001000

  5. Classes of IP addresses All IPv4 addresses have 32 TOTAL bits – 4 octets with 8 bits in each octet. The class determination dictates where the boundary between the network portion and the host portion of address is drawn.

  6. IP Address Class Network and Host Bits 7 14 21

  7. Subnets • As the Internet grew, original class-based addressing scheme proved insufficient • The IPv4 class-based addressing scheme is very inflexible • The choice of network class (size) is limited to either A, B, or C • So, networks support fixed number of hosts: • A) 16.77 million hosts / network • B) 65,534 hosts / network • C) 254 hosts / network

  8. Subnets • Subnetting or classless addressing allows administrator / manager to shift the “standard” class-based boundary between suffix and prefix • (This is a generalized example): the administrator can “break up” 1 class C address space (with 256 addresses) into 4 blocks with 64 addresses in each block

  9. Subnet masks • Subnetting requires the use of an additional piece of information called a subnet mask • This is a separate IP address space that is used along with IP addresses • The subnet mask specifies which bits of an IP address are part of the network ID and which are part of the host ID

  10. Default subnet masks For class B, the first two octets represent the network ID – leaving the last two octets for host identification

  11. Subnets • We are discussing subnets in a somewhat superficial fashion • Assigning host addresses on even small class C subnets is a little more complicated in reality • You can also visit (IP calculator) • http://www.telusplanet.net/public/sparkman/netcalc.htm

  12. Subnetting Example • Standard Class C address space • 1st 3 octets define the network address or the “prefix” (the network identifier) • Last octet defines the host address or the “suffix” within each network address (the host identifier) • Think of the network address as a neighborhood or street reference and the host address as the specific houses on the street

  13. Subnets • How can an admin more effectively allocate address space using a class C IP address? • 254 hosts on standard class C address • Don’t want to give out an entire class C to a subnetwork • If need a max of 24 addresses on the subnet – this would waste 230 addresses • You CANNOT share a network ID between different subnetworks

  14. Subnets • In reality only a few practical subnetting schemes… • 2 bits in prefix, 6 in suffix • 3 bits in prefix, 5 in suffix • 4 bits in prefix, 4 in suffix • EXAMPLE…

  15. Summary • IPv4 address space review • Subnetting conceptual review • Subnetting example

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