IP Addressing and Subnetting

Basics

• modern networks use MAC addresses and IP addresses
• an IP address is a 32-bit number written as 4 numbers (0-255, i.e. 8 bits = 1 octet), separated by dots (called packet address format)
• every IP has two parts: Network ID (in the beginning) and the Host ID (the rest)

Subnet Mask/CIDR

• the subnet mask tells you how many bits belong to the Net ID
• written as "slash" number - /24 - called the Prefix Mask or CIDR notation
• /24 = first 24 bits (3 octets) are the Net ID -> 255.255.255.0
• remaining bits are the Host ID
• sometimes a singe octet is split between Net ID and Host ID (when the prefix isn't a multiple of 8)
• every Host ID must be unique in a network

Assigned Network IDs

• organizations are assigned a Net ID to use (e.g. Champlain College = 216.93.144.0/20)
• all hosts on a network share the same Net ID portion; the network address itself (all 0s host bits) can't be assigned to a device

Subnetting (splitting networks)

• Most organizations need more than one network - using host bits to make new networks helps:

1. prevent one network from getting overcrowded (broadcast traffic issues)
2. improved organization/management
3. enhanced security

• to subnet: borrow bits from the Host ID to create additional Net IDs (e.g. go from /20 -> /24), creates several smaller subnetworks
  • first subnet - 216.92.144.0, second subnet - 216.93.145.0/24
• all 0s in the Host ID = the network address itself -> cannot be assigned to a host
• all 1s in the Host ID = the broadcast address -> cannot be assigned to a host
• smaller networks are easier to manage and less complicated

Class	Range	Default Prefix	Typical Use
A	1.0.0.0 - 127.255.255.255	/8	ISPs, large corporations
B	128.0.0.0 - 191.255.255.255	/16	universities, large businesses
C	192.0.0.0 - 223.255.255.255	/24	small businesses, home networks
D/E	--	--	Exist but not applicable, multicast/experimental

Reserved Ranges

• 127.0.0.0/8 - Loopback (used to test if your own NIC/network stack is working - e.g. 127.0.0.1)
• 169.254.0.0/16 - link-local (auto assigned when no DHCP is available; local hosts only, not routable