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Subnetting

Jun 30, 2024, 3 min read

tags : Networking

IP Addressing

Classes

Classes are just memorization, no math involved here.

ClassRangeExtended RangeDefault CIDR
A1-1261.0.0.1 - 126.255.255.254/8
B128-191128.1.0.1 - 191.155.255.254/16
C192-223192.0.1.1 - 223.255.254.254/24
D (Multicasting)224-239244.0.0.0 - 239.255.255.255N/A
E (Experimental)240-255240.0.0.0 - 254.255.255.254N/A
  • When we classify (A,B,C etc) an IP address, we only look at the 1st Octet.
  • 0.0.0.0 : Default
  • 127.0.0.0-127.255.255.255 : Loopback range (127.0.0.0/8)
  • CIDR has nothing to do with the class of the address
    • You can have class A address with /16 (subnetted)
    • You can have class A address with /24 (subnetted)
  • Has 4 Octets XXXX.XXXX.XXXX.XXXX (32 bits)

Private ranges

Private : Not routable on the internet (NAT comes in)

ClassPrivate RangeDefault CIDR
A10.0.0.0 - 10.255.255.255.255/8
B172.16.0.0 - 172.31.255.255/12
C192.168.0.0 - 192.168.255.255/16

CIDR

Find Range from Mask

  • Example mask: /28
  • Binary: 11111111.11111111.11111111.11110000
  • Network Increment: 16 (Because line gets drawn at 16)
  • Hosts: 15 (0000 : 15)
Network IDBroadcast AddressRange
X.X.X.16X.X.X.3117-30
X.X.X.32X.X.X.4733-46
X.X.X.48X.X.X.6349-62
…

Find Mask from no. of hosts

Start counting from the right to left, then subtract 2(Broadcast+NetworkID)

  • Example no. of hosts: 100
  • 11111111.11111111.11111111.10000000 (Allows 127-2 = 125 Hosts)
  • Subnet : 255.255.255.128
  • /25 will be the Mask. NOTE: Idk why the screenshot starts from 2.

TODO: Get the following checked by someone who understands networking

Network IDB.cast AddrRange
X.X.X.128X.X.X.255129-254

If using 0 network

Network IDB.cast AddrRange
X.X.X.0X.X.X.1271-126
X.X.X.128X.X.X.255129-254

Find Mask for no. of subnets and no.of hosts

Start counting from the left to right, then subtract 2(Broadcast+NetworkID)

  • Example no. of subnets : 300
  • Example no. of hosts : 100
  • 11111111.11111111.11111111.10000000 (Allows 128-2 = 126 Hosts)
  • /25 will be the Mask.

Find Network ID, Range and Broadcast Address from IP&Mask

  • Example IP: 192.168.100.97/27
  • X.X.X.11100000 (/27)
  • Network Increment: 32 (Line gets drawn at 32)
  • Hosts: 31 (00000: 31)
Network IDBroadcast AddressRange
X.X.X.32X.X.X.63
X.X.X.64X.X.X.95
X.X.X.96X.X.X.12797-126
X.X.X.128
…

IPv4 and IPv6 addresses representation

2001:0db8:c9d2:0012:0000:0000:0000:0051
2001:db8:c9d2:12::51

2001:0db8:ab00:0000:0000:0000:0000:0000
2001:db8:ab00::

0000:0000:0000:0000:0000:0000:0000:0001
::1
  • The address ::1 is the loopback address. It always means “this machine I’m running on now”. In IPv4, the loopback address is 127.0.0.1.
  • Other than complete ipv4 and ipv6 address there’s a compatibility mode to represent ipv4 address in ipv6
    • 192.0.2.33 can be written as ::ffff::192.0.2.33

Subnetting Intro

Taking a network and dividing it into sub-networks.

Multihoming vs IP aliasing

  • Multihoming: Host level. When a host is connected to more than one network.
  • IP Aliasing: Interface level. IP aliasing is associating more than one IP address to a network interface.

Point to Point networks

These masks are not supported by every router. Windows doesn’t support it?

  • /30 : 4 addresses (failsafe ptp since this will work with ALL equipment.)
  • /31 : 2 addresses (ptp and tunnel configs)
  • /32 : 1 address (loopback, interfaces, matching hosts in ACLs etc., ISP assigning you an IP)

Resources

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