IPv4 Addressing Schemes

Public IPv4 Address Range

• There are two (2) main IPv4 address space, these are the Public address space and the Private address space.

• The primary difference between both address spaces is the Public IPv4 addresses are routable on the Internet.

• This means any device which requires communication other devices on the Internet will need to be assigned a Public IPv4 address on its interface which is connected to the Internet.

• The public address space is divided into five (5) classes:

• • Class A - 1.0.0.0 – 126.255.255.255

  • Class B - 128.0.0.0 – 191.255.255.255

  • Class C - 192.0.0.0 – 223.255.255.255

  • Class D (Multicast) - 224.0.0.0 – 239.255.255.255

  • Class E (Experimental) - 240.0.0.0 – 255.255.255.255

Issues with using only Public IP addressing

• Large routing table on each device on the Internet.

• Wastage of any unused IP address within a network block.

Private IPv4 Addresses

• There are three (3) classes of Private IPv4 address which are allocated for private use only, this means within a private network such as a local area network (LAN).

• The benefit of using the private address space (RFC 1918), the classes are not unique to any particular organization or group.

• They can be used within an organization or a private network. However, on the Internet, the Public IPv4 address is unique to a device. This means if a device is directly connected to the Internet with a Private IPv4 address, there will be zero network connectivity to devices on the Internet.

• Requires Network Address Translation (NAT) to translate the source Private IPv4 address to the Public IPv4 address. 

• Most Internet Service Providers (ISPs) usually has a filter to prevent any RFC 1918 address from entering their network.

• The private address space is divided into three (3) classes:

• • Class A – 10.0.0.0/8 network block | 10.0.0.0 – 10.255.255.255

  • Class B – 172.16.0.0/12 network block | 172.16.0.0 – 172.31.255.255

  • Class C – 192.168.0.0/16 network block | 192.168.0.0 – 192.168.255.255

• Length - 32 bits in the form of Binary notation

• Decimal - 192.168.1.2

• Binary - 11000000.10110000.00000001.00000011

• Octet – a group of 8 bits

• Converting binary to decimal and vice versa.

Subnet Mask

• For every IP address, there’s an accompanying subnet(work) mask. This address is used to define the following:

• The subnet mask is used to indicate the network and host portion of an IP address.

• The subnet mask is used to determine the number of available IP addresses on a network.

• Identify the Network ID using the ANDing process.

• If two (2) or more devices such as computers are communicating over multiple networks, the subnet mask of each device determines if a computer should send the packet to the default gateway/router or not.

• There are three (3) default classes of subnet mask:

  • Class A = 255.0.0.0

  • Class B = 255.255.0.0

  • Class C = 255.255.255.0

• Each IPv4 address class has their own default subnet mask.

• An IP address such as 10.10.10.1 which is a Private Class A address, will use the subnet mask of 255.0.0.0.

• Another example we can use is the address, 191.5.4.6 which is a Public Class B address, therefore this address will use a default subnet mask of 255.255.0.0.

Network Prefix

• You would have noticed after some IPv4 addresses there’s a forward slash (/) with a number next such as /8, /16 & /24.

• This is the shorthand way of representing a subnet mask. This is known as a network prefix.

• How is a /8, /16 or even a /24 represent an entire subnet mask?

  • Firstly, let’s convert a Class A subnet mask:

  • Decimal: 255.0.0.0

  • Binary: 11111111.00000000.00000000.00000000

  • As we can see, there are eight 1s in the subnet mask, from this denotation, we get /8.

Network Address Translation (NAT)

• Network Address Translation (NAT) is used to map internal private IP address(es) to a public IP address(es) on a router's outside interface.

• When configuring Inside Source NAT, the router's interface which is connected to the WAN/Internet is known as the Outside interface. 

• The interface which is connected to the internal LAN is known as the Inside interface. These terms are used when identifying NAT interfaces.

NAT Terminologies

• Inside Local—This is the local IP address of a private host on your network (e.g., a workstation's IP address).

• Inside Global—This is the public IP address that the outside network sees as the IP address of your local host.

• Outside Local—This is the local IP address from the private network, which your local host sees as the IP address of the remote host.

• Outside Global—This is the public IP address of the remote host (e.g., the IP address of the remote Web server that a workstation is connecting to).

 Types of NAT

•  There are three (3) types of NAT, these are:

  •  Static NAT

  • Dynamic NAT

  • Port Address Translation (PAT)

Static NAT

• Static NAT allows a one-to-one mapping of a private IP address to a public IP address on a router.

• This type of translation/mapping is used provide access to web, email, file servers and so on.

Dynamic NAT

• Dynamic NAT (pool) is used to map multiple private IP addresses to multiple Public IP addresses.

Port Address Translation (PAT)

• In Port Address Translation (PAT), multiple private IP addresses are mapped to a single public IP address.

• PAT is also known as NAT Overload.

• PAT is used to help preserve the public IPv4 address pools, allowing many private IPv4 networks to be mapped onto a single public IPv4 address.