Week 2: Subnetting, VLANS, & Cisco Commands

Week 2: 9/5/22

Subnetting

• Allow very common subnets: /24 (256 IP’s, 254 Usable), /23 (512 IP’s, 510 Usable), /22 (1024 IP’s, 1022 Usable), /30 (4 IP’s, 2 Usable), /29 (8 IP’s, 6 Usable)

• Organizations are “assigned” a network address to use on the internet

  • 216.93.144.0/20

    • All champlain IPs start with the same 20 bits

    • Last 12 bits are used for host ID

    • 2^12 IPs can be used

      • All zeros and all 1’s in the host ID can’t be used

• But a /20 network cna support 4094 hosts - do we want them all on the same network?

  • Lots of broadcast packets congest the network

  • Machines are slowed by trying to process them

  • Anyone can contact anyone else on the network!

• Our network ID can’t change but what if we took some host ID bits to create a subnet ID

  • These bits are then ”added” to the network ID

  • For example if we used 4 bits for the subnet ID

• Wireless networks are normally larger networks because most devices are looking for information outside the network. Larger network means less communication inside the network.

• Smaller networks are found in places where devices communicate within the network. For example a data center. (Network to Network firewall rules instead of Host to Host rules)

• Always start with the largest subnet first: Larger subnet boundaries are always valid for smallers ones, but smaller boundaries are not always valid for larger ones

• All 0’s in the Host ID refers to the Network itself - cannot be assigned to a host

• All 1’s in the Host ID is the broadcast -cannot be assigned to a host

• So if we have n bits in our Host ID, we can assign 2^2 -2 IP addresses

VLANS

• Multilayer switching

• A virtual LAN is a group of devices on one or more physical LANs that are configured to communicate as if they were on the same LAN.

• VLANs define broadcast domains in a Layer 2 network

• Need a router to pass packets between 2 different VLANs

• Access Ports: Can only be assigned to/carry traffic from a single VLAN

  • Used to connect end device to a switch

• Trunk Ports: Carry traffic from multiple VLANs - used to connect switches

  • Will “tag” packets with the proper VLAN ID

CISCO IOS Config File

• Startup-config: This is the last version that was saved. If the router/switch was restarted, this is the config that would load

• Running-config: Whats running in memory. It has all of the config changes made at the command line since it was last saved.

• User EXEC Mode: Basic monitoring and status commands

  • Prompt it: router>

  • Type enable to get to Privileged

• Privileged EXEC Mode: Administrative access

  • Prompt: router#

  • Type config to get to the config mode

• Config EXEC MODE: config mode

  • Prompt: router(config)#

• The question mark ?:

  • Just hit enter and it will show everything you can do

• Tab works in IOS