DRAFT

A public IP network is reachable from the public internet. It uses public IP addresses assigned by the ISP. These addresses are unique (they can’t be reused by someone else on the Internet). Public IP addresses are used by web servers, email servers, and any device or service that needs to be directly accessible on the public internet.

Private IP addresses are used within private, internal networks and are not routable on the public internet. These addresses can be reused in different private networks. They are reserved for local communication within an organization.  If you want to make a resource accessible from the internet, you need to use NAT.

You can choose whatever private IP addresses in the class A, B, or C. 

Class A: 10.0.0.0 to 10.255.255.255

Class B: 172.16.0.0 to 172.31.255.255

Class C: 192.168.0.0 to 192.168.255.255

We can only use either Public addresses we bought or Private addresses.

If a company uses a class A/B/C public address which does not belong to them, they will have access to the Internet. However, if they try to access a site or a resource in this address range, its router will redirect the requests internally. The company will not be able to access these resources.

 

Former IP addresses class

Class A mask 255.0.0.0

Available addresses (MSB is 0):

Public Network: 1.0.0.0 - 126.0.0.0. Subnet 255.0.0.0 (or /8) (126 networks - 124 that can communicate with the web)

Private network: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255) (1 private network, cannot communicate with the web)

Local loopback test reserve: 127.0.0.0 (cannot communicate with the web)

 

Class B mask 255.255.0.0

Available addresses (MSB is 1, next is 0):

Public network: 128.0.0.0 - 191.255.0.0 Subnet 255.255.0.0 (or /16) (13684 networks -17 cannot communicate with the web)

Private network: 172.16.0.0/28 (172.16.0.0 - 172.31.0.0) (16 networks cannot communicate with the web)

APIPA reserve (LINK-LOCAL): 169.254.0.0 (cannot communicate with the web)

 

Class C mask 255.255.255.0

Available addresses (MSB is 1, next 1, next 0) :

Public network: 192.0.0.0 - 223.255.255.255 Subnet 255.255.255.0 (or /24) (256 cannot communicate with the web)

Reserve: 192.0.2.0 - 192.0.2.255 (cannot communicate with the web)

Private network: 192.168.0.0/16 (192.168.0.0 - 192.168.255.0) (255 networks cannot communicate with the web)

 

Other reserved addresses :

224.0.0.0 - 239.0.0.0 Multicast (cannot communicate with the web)

240.0.0.0 - 255.255.255.254 Experimental (cannot communicate with the web)

 

CIDR method and prefix notation

CIDR stands for Classless Inter-Domain Routing. This is a prefix notation system used to specify IP addresses along with their associated subnetting information. For example : 192.168.1.99/24 specifies that the first 24 bits of the address are the network portion (255.255.255.0), leaving 8 bits for host addresses. In this case, we know that the network address is 192.168.1.0.

 

CIDR uses Variable-Length Subnet Masks (VLSM):

It allows for the use of subnet masks of varying lengths, rather than being restricted to the fixed subnet masks defined by the former IP address classes. It is often referred to as "classless" because it does not rely on the traditional Class A, B, and C divisions. This flexibility enables more efficient use of IP address space, as it permits the allocation of smaller address blocks as needed, rather than assigning larger, fixed-size blocks as was the case with IP address classes. 

The rapid growth of the internet made it evident that more addresses would be needed to connect devices than the IPv4 address space had available. IPv6 was developed to address this issue of IPv4 address exhaustion.  

IPv6 uses 128-bit addresses, whereas IPv4 uses 32-bits. While IPv4 is still widely used, the adoption of IPv6 has been steadily increasing.

Address Format: IPv6 addresses are 128-bit numerical labels, typically represented in hexadecimal notation (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).

IPv6 is designed to provide each device with a globally unique address, eliminating the need for Network Address Translation (NAT) in most cases.