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  1. Introduction
  2. OSI Layers
  3. Addresses
  4. SNMP
  5. Network Tools
  6. Routing
  7. ARP
  8. WINS
  9. DNS
  10. NIS
  11. DHCP
  12. Name Resolution
  13. RAS
  14. Connections
  15. Test Pointers
  16. Items to remember
  17. Terms
  18. Credits

TCP/IP Addresses

IP addresses are broken into 4 octets (IPv4) separated by dots called dotted decimal notation. An octet is a byte consisting of 8 bits. The IPv4 addresses are in the following form:

There are two parts of an IP address:

  • Network ID
  • Host ID

The various classes of networks designate more or less octets to designate the network ID versus the host ID.

Class1st Octet2nd Octet3rd Octet4th Octet
Net IDHost ID
Net IDHost ID
Net IDHost ID

Leading bit patterns for classes of networks:

ClassPatternMax NetworksMax Nodes

Subnet masks

Subnetting is the process of breaking down a main class A, B, or C network into subnets for routing purposes. When doing subnetting, the number of bits in the subnet mask determine the number of available subnets. Two to the power of the number of bits minus two is the number of available subnets. When setting up subnets the following must be determined:

  • Number of segments
  • Hosts per segment

Subnetting provides the following advantages:

  • Network traffic isolation - There is less network traffic on each subnet.
  • Simplified Administration - Networks may be managed independently.
  • Improved security - Subnets can isolate internal networks so they are not visible from external networks.

A 14 bit subnet mask on a class B network only allows 2 node addresses for WAN links. A routing algorithm like OSPF or EIGRP must be used for this approach. These protocols allow the variable length subnet masks (VLSM). RIP and IGRP don't support this. Subnet mask information must be transmitted on the update packets for dynamic routing protocols for this to work. The router subnet mask is different than the WAN interface subnet mask.

One network ID is required by each of:

  • Subnet
  • WAN connection

One host ID is required by each of:

  • Each NIC on each host.
  • Each router interface.

Types of subnet masks:

  • Default - Fits into a Class A, B, or C network category
  • Custom - Used to break a default network such as a Class A, B, or C network into subnets.


IPv6 is 128 bits. It has eight octet pairs, each with 16 bits and written in hexadecimal as follows:


Extension headers can be added to IPv6 for new features.


Supernetting is used to help make up for some of the shortage if IP addresses for the internet. It uses Classless Inter-Domain Routing (CIDR). If a business needs a specific number of IP addresses such as 1500, rather than allocating a class B set of addresses with the subnet mask of, a subnet mask of may be allocated. Therefore the equivalent of eight class C addresses have been allocated. With supernetting, the value of 2 is not subtracted from the possible number of subnets since the router knows that these are contiguous networks. 8 times 254 = 2032.