# Internet Layer
## IPv4 Addressing

IPv4 Addressing refers to an address that conforms to the Version 4 standard of the Internet Protocol (IP). These addresses are assigned to computers to identify them on an IP Network. IPv4 addresses are 32-bits long, with 4 octets of 8 bits each. While IP Addresses are binary numbers, IP Addresses are usually written in what is called a dotted decimal notation so that they are human readable. Dotted Decimal notation is accomplished by converting the binary values of each octet to its decimal number equivilent and placing a decimal point between each octet (i.e. 159.203.155.100).

### Dotted Decimal Notation

Like the IPv4 Address itself, Subnet Masks are sometimes represented in dotted decimal notation.

### CIDR Notation

Classless Interdomain Routing (CIDR) Notation refers to using a two to three character representation of a subnet mask. CIDR is written as a slash (/) and then a number that represents the number of bits used in the subnet mask. For example, if a subnet mask in binary format is 11111111111111110000000000000000, there are 16 total on bits, therefore the CIDR Notation for this mask would be /16.
In Cisco CIDR is mostly used to represent subnets and can normally be seen as the following /8, /16, and 24/. The more the network bits positions the fewer the hosts and the more the hosts bits positions the more the networks.

## Classes

* Class A
** Contains the IPv4 Addresses 0.0.0.0-127.255.255.255. Default subnet mask is 255.0.0.0.
* Class B
** Contains the IPv4 Addresses 128.0.0.0-191.255.255.255. Default subnet mask is 255.255.0.0.
* Class C
** Contains the IPv4 Addresses 192.0.0.0-223.255.255.255. Default subnet mask is 255.255.255.0.
* Class D
** Contains the IPv4 Addresses 224.0.0.0-239.255.255.255. This address is for multicasting.
* Class E
** Contains the IPv4 Addresses 240.0.0.0-255.255.255.255. These addresses are reserved by the Internet Assigned Numbers Authority (IANA) and should not be used on IP Networks.

### Loopback Addresses

The 127.0.0.0/8 network is a part of the Class A classification, but it is designated for loopback addressing and cannot be assigned to a network. The loopback subnet is not routable on the internet. Its mainly for troubleshooting of the NIC and can be mixed up to different IP addresses using class A IP addresses because class A support about 16 million hosts. eg. 127.1.1.1, 127.54.1.7, 127.3.1.254 can all be used as loopback addresses.

### Private Addresses

There are 3 IPv4 address ranges that are considered private addresses, and these ranges are:
* Class A: 10.0.0.0/8
* Class B: 172.16.0.0-172.31.0.0/16
* Class C: 192.168.0.0/16
These addresses are not routable on the internet, meaning that in order for computers that are assigned a private address to communicate with the internet, a Network Address Translation (NAT) service must be put in place between the privately addressed computer and the internet.
Private IP address are very essential because it helps to avoid hackers to invade or attack our local intranet. However public IP addresses are routable on the internet and can therefore been easily tracked and attacked by hackers.

### Subnetting

Subnetting describes the process of dividing IP Addresses into logical subnetworks.

## See Also

* Internet Protocol Analysis - Internet Layer / IPv4
* Internet Protocol Analysis - Internet Layer / Subnetting