CCNA-Notes/13. Subnetting (Part 1).md

id, aliases, tags

id	aliases	tags
1777840095-WYOD	Subnetting (Part 1)	CCNA

Subnetting (Part 1)

IPV4 Address Classes

Class	First octet	numeric range	Prefix Length
A	0xxxxxxxx	0-127	/8
B	10xxxxxxx	128-191	/16
C	110xxxxxx	192-223	/24
D	1110xxxxx	224-239
E	1111xxxxx	240-255

Maximum Hosts per Network

Host portion all 0s = Network address (network ID) Host portion all 1s = broadcast address

192.168.1.0/24 -> 192.168.1.255/24 Host portion = 8 bits = 2^8 = 256 Maximum hosts per network = 2^8-2 = 254

172.16.0.0/16 -> 172.16.255.255/16 Host portion = 16 bits = 2^16 = 65,536 Maximum hosts per network = 2^16 -2 = 254

10.0.0.0/8 -> 10.255.255.255/8 Host portion = 24 bits = 2^24 = 16,777,216 Maximum hosts per network = 2^24 -2 = 16,777,214

How IP addresses are distrubuted

• The IANA (Internet Assigned Numbers Authority) assigns IPV4 addresses/networks to companies based on their size
• For example, a very large company might receive a class A or class B network, while a small company might receive a ckass C network.
• However, this led to many wasted IP addresses.

Example 1

+----+ +----+ | R1 |<----->| R2 | +----+ ^ +----+ Point to Point network

• 256 addresses
• 1 network address (203.0.113.0)
• 1 broadcast address (203.0.113.255)
• 1 R1 address (203.0.113.1)
• 1 R2 address (203.0.113.2)
• 252 addresses Wasted

Example 2

Company X needs IP addressing for 5000 end hosts.

• A class C network does not provide enough addresses, so a class B network must be assigned
• This will result in about 6000 addresses being wasted.

CIDR

• When the internet was first created, the creators did not predict that the internet would become as large as it is today
• This resulted in wasted address space like the examples.
• The IETF introduced CIDR in 1993 to replace the 'classful' addressing system.
• With CIDR, the requirement of ...
  • Class A = /8
  • Class B = /16
  • Class C = /24 were removed
• This allowed larger networks to be split into smaller networks, allowing greater efficiency
• These smaller networks are called 'subnetworks' or subnetting

+----+ +----+ | R1 |<----->| R2 | +----+ ^ +----+ 203.0.113.0/24

Network address 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0

Subnet Mask 11111111.11111111.11111111.00000000 255 . 255 . 255 . 0

2^8 - 2 = 254 usable addresses.

How many usable addreses are there in each network?

CIDR /25

203.0.113.0/25 Network address: 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0 Subnet Mask 11111111.11111111.11111111.10000000 255 . 255 . 255 . 128 2^7 - 2 = 126 usable addresses

CIDR /26

203.0.113.0/26 Network address 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0 Subnet Mask 11111111.11111111.11111111.11000000 255 . 255 . 255 . 192 2^6 - 2 = 62 usable addresses

CIDR /27

203.0.113.0/27 Network address 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0 Subnet Mask 11111111.11111111.11111111.11100000 255 . 255 . 255 . 224 2^5 - 2 = 30 usable addresses

CIDR /28

203.0.113.0/28 Network address 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0 Subnet Mask 11111111.11111111.11111111.11110000 255 . 255 . 255 . 240 2^4 - 2 = 14 usable addresses

CIDR /29

203.0.113.0/29 Network address 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0 Subnet Mask 11111111.11111111.11111111.11111000 255 . 255 . 255 . 248 2^4 - 2 = 6 usable addresses

CIDR /30

203.0.113.0/30 Network address 11001011.00000000.01110001.00000000 203 . 0 . 113 . 0 Subnet Mask 11111111.11111111.11111111.11111100 255 . 255 . 255 . 252 2^4 - 2 = 6 usable addresses

This is the perfect CIDR for our 2 router

The remaining addresses in the 20 203.0.113.0/24 address block (20 203.0.113.4 - 20 203.0.113.255) are now available to be used in other subnets!

CIDR /31

203.0.113.0/31

2^1 -2= 0 usable addresses

but for our point to point connection we can utilisize it no need for broadcast or network address In cisco device however you get a systeme warning

Router(config-if)# ip address 203.0.113.0 255.255.255.254
Warning use /31 mask on non point-to-pint interface cautiously

The remaining addresses in the 20 203.0.113.0/24 address block (20 203.0.113.2 - 203.0.113.255) are now available to be used in other network!

CIDR /32

203.0.113.0/32

2^1 -2= -1 usable addresses

can be used in static routes don't worry about them now

Review

• CIDR (Classless Inter-Domain Routing)
• The process of subnetting