2Chapter IP Addressing, Subnetting

• IP Addressing


  An address used in order to uniquely identify a device on a computer network
• An IP address is an identifier for a computer or device on a TCP/IP network
• Networks use the IP address of the destination to route messages
• IP address


  An identifier that is assigned at the Internet layer to an interface or a set of interfaces
• Each IP address can identify the source or destination of IP packets
• When you enable TCP/IP on an interface, you assign it one or more logical IP addresses, either automatically or manually
• The IP address is a logical address because it is assigned at the Internet layer and has no relation to the physical addresses
• IPv4


  The current version of IP, defines a 32-bit address which means that there are only 2^32 (4,294,967,296) IPv4 addresses available
• The address shortage problem is aggravated by the fact that portions of the IP address space have not been efficiently allocated
• The traditional model of classful addressing does not allow the address space to be used to its maximum potential
• Dotted-decimal notation


  IP addresses are often expressed as four decimal numbers, each separated by a dot to make them easier for human users to read and write
• Converting IPv4 address from binary to dotted decimal

  1. Segment it into 8-bit blocks

  2. Convert each block to decimal

  3. Separate the blocks with periods
• Classful addressing


  The IP address space is split into three predefined classes (A, B, C) with each class fixing the boundary between the network-prefix and the host-number at a different point within the 32bit address
• Class A addresses


  • 8-bit network-prefix with the highest order bit set to 0 and a seven-bit network number, followed by a 24-bit host number
  • Maximum of 126 /8 networks can be defined
  • Each /8 supports a maximum of 16,777,214 hosts per network
• Class B addresses


  • 16-bit network-prefix with the two highest order bits set to 1-0 and a 14-bit network number, followed by a 16-bit host-number
  • Maximum of 16,384 /16 networks can be defined
  • Each /16 supports up to 65,534 hosts per network
• Class C addresses


  • 24-bit network-prefix with the three highest order bits set to 1-1-0 and a 21-bit network number, followed by an 8-bit host-number
  • Maximum of 2,097,152 /24 networks can be defined
  • Each /24 supports up to 254 hosts per network
• Class D addresses


  Reserved for IPv4 multicast addresses, the four high-order bits are always set to 1110
• Class E addresses


  Reserved for experimental use, the high-order bits are set to 1111
• The classful A, B, and C octet boundaries were easy to understand and implement, but they did not foster the efficient allocation of a finite address space
• ICANN assigns public addresses, which consist of either historically allocated classful address prefixes or CIDR address prefixes
• When ICANN assigns a public address prefix to an organization, routes are added to the routers of the Internet so that traffic matching the address prefix can reach the organization
• Private organization intranets that do not need an Internet connection can choose any address scheme they want, even using public address prefixes that ICANN has assigned to other networks
• If the private organization later decides to directly connect to the Internet, these addresses could conflict with existing public addresses and become illegal addresses
• Private addresses


  IPv4 addresses in the private address space that are never assigned as public addresses, so they never duplicate public addresses
• Private address space


  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16
• Subnetting


  The practice of dividing a network into two or more smaller logical networks
• Subnetting


  Designating some high-order bits from the host part and grouping them with the network part to divide a network into smaller subnets
• Subnetting ensures that the subnet structure of a network is never visible outside of the organization's private network
• The route from the Internet to any subnet of a given IP address is the same, no matter which subnet the destination host is on
• Routers within the private organization need to differentiate between the subnets
• Subnetting


  Logical subdivision of an IP network into smaller subnets
• Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network
• If you do not subnet, you are only able to use one network from your Class A, B, or C network, which is unrealistic
• Subnetting


  1. Designating some high-order bits from the host part and grouping them with the network part

  2. Dividing a network into smaller subnets
• Subnetting


  • Ensures that the subnet structure of a network is never visible outside of the organization's private network
  • Allows the local administrator to introduce arbitrary complexity into the private network without affecting the size of the Internet's routing tables
  • Overcomes the registered number issue by assigning each organization one (or at most a few) network number(s) from the IPv4 address space
• Details of Subnetting


  • Prefix size
  • Network mask
  • Available subnets
  • Usable hosts per subnet
  • Total usable hosts
• Extended-network-prefix


  Composed of the classful network-prefix and the subnet-number
• Subnet mask


  Identifies the extended-network-prefix
• Prefix length


  Equal to the number of contiguous one-bits in the traditional subnet mask
• VLSM (Variable Length Subnet Mask)

  The process of "subnetting a subnet" and using different subnet masks for different networks in an IP plan