Subnets and NAT

Basics

Subnetting is when you take one network address range and create multiple subnetworks within that range. Benefits include:

• Reduced network traffic: routers keep most traffic on the local network, only packets destined for that network travel through its router. Each router creates a broadcast domain, so the more broadcast domains you create, the smaller the less network traffic on each segment.
• Optimized network performance: Reward for reducing network traffic
• Simplified management: Easier to ID and isolate network problems in a group of smaller connected networks than within one giant network
• Facilitated spanning of large geographical distances: Connecting multiple smaller networks is more efficient bc data doesn’t have to travel as far

Create a subnet

To create a subnet, you take bits from the host portion of the IP address and resever them to define the subnet address. Use the following process to determine your needs:

1. Determine the number of network IDs
   • one for each subnet
2. Determine the number of hosts per subnet:
   • each TCP/IP host
   • each router interface
3. Using info from previous steps, create the following:
   • One subnet mask for the entire network
   • Unique subnet ID for each physical segment
   • Range of host IDs for each subnet

Remember powers of 2:

• Each successive power of 2 is double the previous one.
• For example, 2^3 = 8, so 2^4 = 16

Subnet masks

A 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address

• Network admin creates a 32-bit subnet mask of 1s and 0s
• 1s represent the network (or subnet) addresses

Default subnet masks

Classless Inter-Domain Routing (CIDR)

Method that ISPs use to allocate a number of addresses to a company or home connection.

• provided addresses in a block size
• For example: 192.168.10.32/28
  • /XX is the number of bits turned on (1). So 28 is the network
  • Most available is /30 because you need to keep at least 2 bits for network and broadcast addresses
  • So 255.0.0.0 is a a /8 because all bits in the first octet are on

All subnets and CIDR vals

Subnetting Class C

Class C only has 8 bits available for defining hosts:

Quick subnetting

Answer the following questions:

• How many subnets does the chosen subnet mask produce
  • 2^x == number of subnets, where x is the number of 1s in the subnet mask. 11100000 gives 8 subnets, bc 2^3 = 8.
• How many valid hosts per subnet are available
  • 2^y - 2 == number of hosts, where y is the number of 0s in the subnet mask. 11100000 gives 30 hosts, because 2^5 = 32, and 32 - 2 = 30. You subtract 2 for the broadcast and network addresses.
• What are the valid subnets?
  • ???????????????
• What’s the broadcast addr of each subnet?
  • The broadcast address is always the number right before the next subnet. The 64 subnet has a broadcast address of 127 because the next subnet starts at 128.
• What are the valid hosts in each subnet?
  • All numbers between the subnets, omitting the 0s and 1s. If 64 is the subnet number and 127 is the broadcast, so valid addresses are 65 - 126.