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Subnetting

IPv4 Classes

Class First Octet decimal (range) First Octet binary (range) IP Range Subnet Mask Hosts per Network ID # of Networks
Class A 0-127 0XXXXXXX 0.0.0.0 - 127.255.255.255 255.0.0.0 224-2 27
Class B 128-191 10XXXXXX 128.0.0.0 - 191.255.255.255 255.255.0.0 216-2 214
Class C 192-223 110XXXXX 192.0.0.0 - 223.255.255.255 255.255.255.0 28-2 221
Class D (Multicast) 224-239 1110XXXX 224.0.0.0 - 239.255.255.255
Class E (Experimental) 240-255 1111XXXX 240.0.0.0 - 255.255.255.255
h = 2x-2 n = 2y
  • h = 2x-2 ; x is the number of 0's (in binary) in the subnet mask
  • n = 2y ; y is the number of 1's (in binary) in the subnet mask - only including unfixed values.
    • The fixed values in the First Octet (in binary) are not counted towards y

Subnet Masks

https://www.youtube.com/watch?v=yLeuGOOrUvo&list=PLSNNzog5eydt_plAtt3k_LYuIXrAS4aDZ&index=4

  • Why do we need them?
    • Indicates which devices are local vs remote.
  • How?
    • Compare Device IPs (in binary) where Subnet Mask (in binary) == 1
    • Subnet Mask indicates which binary values from each device's IP should be used to decide if devices are local/remote.

Example

  • Device A Subnet Mask: 255.255.255.0
  • Device A IP Address: 10.1.151.2
  • Device B IP Address: 10.1.151.3
  • Device C IP Address: 64.227.160.23

Convert Subnet Masks and IPs into Binary

Label 1st Octet 2nd Octet 3rd Octet 4th Octet
A's Subnet Mask 11111111 11111111 11111111 00000000
A's IP Address: 00001010 00000001 10010111 00000010
B's IP Address: 00001010 00000001 10010111 00000011
Compare A to B Matches Matches Matches N/A - Subnet Mask is 0
C's IP Address: 01000000 11100011 10100000 00010111
Compare A to C Doesn't Match Doesn't Match Doesn't Match
  • Device A and B are on same network.
  • Device A and C are on different networks.

Remote vs Local Protocol

  • Device A wants to communicate with Device B (local)
    1. A uses ARP to ask for B's MAC Address via B's IP
    2. B replies with B's MAC Address
    3. A uses B's MAC to make Frames and communicate with B
    4. All communication between A & B via Switch (layer 2 Device)

  • Device A wants to communicate with Device C (remote network)

    1. A uses ARP to ask for Default Gateway's MAC Address based on Default Gateway's IP address.
    2. Default Gateway replies to A with Default Gateway's MAC Address
    3. A sends packets (for C) to Default Gateway's MAC Address, which delivers A's packets to remote computer C

  • ARP used in both Remote and Local communications

  • IP Address used for remote communications
  • MAC Address used for local communications
  • Switch (layer 2 device) used for Local communications
  • Default Gateway (layer 3 device) used for Remote communications

Subnet Shorthand

Shorthand is the count of 1's in the binary form of the subnet mask.

Shorthand Binary Decimal
/8 11111111.00000000.00000000.00000000 255.0.0.0
/16 11111111.11111111.00000000.00000000 255.255.0.0
/5 11111000.00000000.00000000.00000000 248.0.0.0
/20 11111111.11111111.11110000.0000 255.255.240.0
/25 11111111.11111111.11111111.10000000 255.255.255.128

Subnetting Table
Subnet 1 2 4 8 16 32 64
Host 256 128 64 32 16 8 4
Subnet Mask /24 /25 /26 /27 /28 /29 /30

Example Problems

Example 1

  • IP Address Given: 192.168.1.0
  • Hosts Needed: 60
  • Subnets Needed: 4
                      ___
SUBS:            2 |   4 |   8 |  16 | 32 |  64 | 128 | 256
192.168.1.X:   128 |  64 |  32 |  16 |  8 |   4 |   2 |   1
HOST:          256 | 128 |  64 |  32 | 16 |   8 |   4 |   2
                            ^^
CLASS: C                                     {    HOST IPS   }
DEFAULT SNM: /24     192.168.1 { .0    +1 => | .1   <-> .62  | <= -1 .63  }
CUSTOM SNM: /26                { .64   +1 => | .65  <-> .126 | <= -1 .127 } BROAD
HOSTS(#-2): 62             NET { .128  +1 => | .129 <-> .190 | <= -1 .191 } CAST
SUBNETS: 4                     { .192  +1 => | .192 <-> .254 | <= -1 .255 }
                               { .256
                                  ^^^ Invalid