Understanding IP Addresses

• TCP/IP configuration on a Windows computer requires an IP address, subnet mask, and default gateway.

IP Addresses

• An IP address uniquely identifies a device on an IP network.
• IP address consists of 32 binary bits, divided into a network portion and a host portion using a subnet mask.
• The 32 bits are broken into four octets, where 1 octet equals 8 bits.
• Each octet is converted to decimal and separated by a dot, expressed in dotted decimal format e.g. 172.16.81.100.
• Octet values range from 0 to 255 in decimal, or 00000000 to 11111111 in binary
• An IP address is a 32-bit number that uniquely identifies a host on a TCP/IP network.
• The rightmost bit (least significant bit) of an octet has a value of 20, while the leftmost bit (most significant bit) has a value of 27 when converting binary octets to decimal.
• Where all binary bits are 1, the decimal value is 255; e.g., 11111111 = 128+64+32+16+8+4+2+1=255
• Octet conversion example: 01000001 = 0+64+0+0+0+0+0+1=65
• IP addressed in binary and decimal example: 10.1.23.19 (decimal) = 00001010.00000001.00010111.00010011 (binary).
• These octets are divided to provide an addressing scheme for networks of varying sizes.
• There are five network classes (A to E), this focuses on classes A to C.

IP Classes

• Class A addresses have the first octet as the network portion, with a major network address range of 1.0.0.0 to 127.255.255.255.
• The next 24 bits (octets 2, 3, and 4) are for dividing into subnets and hosts, suitable for networks with over 65,536 hosts (up to 16,777,214).
• Class B addresses have the first two octets as the network portion, with a major network address range of 128.0.0.0 to 191.255.255.255.
• 16 bits (octets 3 and 4) are for local subnets and hosts, suitable for networks with 256 to 65,534 hosts.
• Class C addresses have the first three octets as the network portion, with a major network address range of 192.0.0.0 to 223.255.255.255
• Octet 4 (8 bits) is for local subnets and hosts, suitable for networks with less than 254 hosts.

Network Masks

• A network mask identifies the network and node portions of an address, Class A, B, and C networks have default masks also known as natural masks.
• Class A: 255.0.0.0
• Class B: 255.255.0.0
• Class C: 255.255.255.0
• A Class A network without subnetting has an address/mask pair, such as 8.20.15.1 255.0.0.0.
• The address and mask should be converted to binary to see how the mask helps identify the network and node parts of the address.
• Address and mask example:
  • 8.20.15.1 = 00001000.00010100.00001111.00000001
  • 255.0.0.0 = 11111111.00000000.00000000.00000000
• Address bits with corresponding mask bits set to 1 represent the network ID, while bits with corresponding mask bits set to 0 represent the node ID.
• Based on the previous binary conversions:
  • Net ID = 00001000 = 8
  • Host ID = 00010100.00001111.00000001 = 20.15.1