Private And Public Address

Private And Public Address

RFC 1918 defines three different IP address ranges as private, meaning that they can be used by any private network for internal use, and these ranges are not to be used in the Internet.
Addresses that cannot be routed through the public Internet are private IP addresses.

Private address Ranges:

Private class A IP addresses: 10.0.0.0 to 10.255.255.255
Private class B IP addresses: 172.16.0.0 to 172.31.255.255
Private class C IP addresses: 192.168.0.0 to 192.168.255.255
Rest of the addresses of Class A, B and C are public addresses.


Class D addresses are reserved for multicast and can't be routed across the Internet and class D IP addresses begin with 224.0.0.0.
Also we can't use 127.x.x.x address because the number 127 is reserved for loopback and is used for testing local machine.
Don’t confuse with range of these classes.

Public address Ranges:

Class A: 1.0.0.0 - 126.255.255.255(1 to 126)
Class B: 128.0.0.0 - 191.255.255.255(128 to 191)
Class C: 192.0.0.0 - 223.255.255.255(192 to 223)
Class D: 224 to 239
Class E: 240 to 255

We can summarize above data in a table.

Class	Private IP Address Range	Public IP Address Range
Class A	10.0.0.0 – 10.255.255.255	1.0.0.0 – 9.255.255.255 11.0.0.0 – 126.255.255.255
Class B	172.16.0.0 – 172.31.255.255	128.0.0.0 – 172.15.255.255 172.32.0.0 – 191.255.255.255
Class C	192.168.0.0 – 192.168.255.255	192.0.0.0 – 192.167.255.255 192.169.0.0 – 223.255.255.255

Example 1:

Which addresses should not be advertised to the Internet?
A.172.12.0.1
B.192.168.1.2
C.10.0.78.22
D.112.56.20.41
E.127.0.0.1

Solution:

  B C E.

  Any private addresses (and local addresses) should  not be
  advertised to the Internet.
  B 192.168.1.2 part of the  192.168.0.0/24 which are private
  address of Class C.
  C 10.0.78.22  part of the  10.0.0.0/8 which are private
  address of Class  A.
  E 127.0.0.1  (loopback address).

Example 2:

Which of the following IP addresses are not valid for host usage? ( Choose the three best Solutions).
A.10.0.255.1/8
B.192.20.75.95/27
C.200.254.7.193/28
D.127.0.0.1
E.137.10.1.255/24

Solution:

B, D, E
Option A is a broadcast address.
Option D is a loop back address.
Option E is a broadcast address.

Example 3:

Which of the following addresses are member of networks that can be routed across the public Internet? (Choose four).
A.200.168.48.254
B.172.16.20.15
C.172.64.12.29
D.127.168.23.52
E.198.234.12.95
F.212.193.48.254

Solution:

  A, C, E, F
  Addresses  that can be routed across the public Internet 
  are called public IP addresses.  These addresses belong
  to class A, B and C only and are not private addresses.