What is Private IP and Public IP? where to use each? How can get each? and what the purpose these terms?

A public IP address is any valid address, or number, that can be accessed over the Internet

A private IP address is any number or address assigned to a device on a private TCP/IP Local Area Network that is accessible only within the Local Area Network

Public IP addresses

A public IP address is any valid address, or number, that can be accessed over the Internet.  Internet standards groups, such as the Network Information Center (NIC) or the Internet Assigned Numbers Authority (IANA), are the organizations responsible for registering IP ranges and assigning them to organizations, such as Internet Service Providers (ISPs).

In the Cloud (n) system, a public IP address is an identifier assigned to a virtual router on the network. Any resources that will be available over the Internet will require a public IP address.  Public IP addresses can be added in the Cloud Console.

Private IP addresses

A private IP address is any number or address assigned to a device on a private TCP/IP Local Area Network that is accessible only within the Local Area Network.  For a resource inside the Local Area Network to be accessible over the Internet, a device within the Local Area Network must be connected to the Internet with a public IP address, and the networking must be appropriately configured.  The same Internet standards organizations have reserved the following three IP address ranges that will never be registered publicly:

First IP in block

Last IP in block

10.0.0.0

10.255.255.255

172.16.0.0

172.31.255.255

192.168.0.0

192.168.255.255

A private IP address is assigned to each instance created in the Cloud(n) system.  Consequently, each instance may only have one private IP address, and additional private IP addresses cannot be added.

Both exist for the same reason: to provide unique identification to every device on a network.

Public (external) IP addresses

A public (or external) IP address is the one that your ISP (Internet Service Provider) provides to identify your home network to the outside world. It is an IP address that is unique throughout the entire Internet.

Depending on your service, you might have an IP address that never changes (a fixed, or static IP address). But most ISPs provide an IP address that can change from time to time (a dynamic IP address). For the vast majority of users, a dynamic IP address is fine.

When you're setting up your router, if your ISP issued you a static IP address, you enter it into your router's settings. For a dynamic IP address, you specify DHCP in your router's network settings. DHCPis Dynamic Host Control Protocol. It tells your router to accept whatever public IP address your ISP issues.

Private (internal) IP addresses

Just as your network's public IP address is issued by your ISP, your router issues private (or internal) IP addresses to each network device inside your network. This provides unique identification for devices that are within your home network, such as your computer, your Slingbox, and so on.

Similar to the arrangement with public IP addresses, each device on your network has its network configuration settings on DHCP, so it can accept the unique private IP address that your router issues it.

These private IP addresses never leave your network, just as your public IP address is never used inside your network. The router controls all the network traffic, both within your home network and outside of it, to the Internet. It is the router's job to make sure that data flows to and from all the correct places.

Every device connected to a network needs an IP, and all IPs in a local network need to be in the same subnet. when a user tries to send a packet the NIC reacts as the following:

1- is the destination IP in the same subnet with my IP?

2- if yes send the packet to the destination IP immediatly (by getting the destination mac address using ARP protocol)

3- if No send the packet to the default gateway (which should be the router if the network is configured properly)

That behavior can create one problem only, if I choose a subnet for my local network then I try to access a website that happens to have an IP address in the same subnet, the NIC will think it is in my local network and will never send the packet to the router, which means that website is never accessable to me. To solve that problem, an organization called IANA which is the one responsible for IP assigning over the internet, chose some IPs and made sure they are never assigned to websites on the internet, these IPs are called private IPs, all other IPs are called Public IPs. any user can assign private IPs in his/her private network at home or at work and be sure that he/she can access all internet websites with no problems.

Private IPs are:10.0.0.0 -10.255.255.255,172.16.0.0 -172.31.255.255,192.168.0.0 -192.168.255.255 

Private Addresses

Private IP addresses that are designated for networks that have limited or no access to the Internet. Hosts or packets using these addresses as a source and destination are not to appear on the public Internet.

 These private address blocks are:

10.0.0.0 –10.255.255.255 (10.0.0.0 /8)

10.0.1.0 –172.16.0.0 to172.16.255.255 (172.16.0.0 /12)

10.0.2.0 –192.168.0.0 to192.168.255.255 (192.168.0.0 /16)

Public Addresses

Most of the addresses in the IPv4 host range are public addresses. These addresses are designed for used by hosts that are publicly accessible from the Internet. Even within these address blocks, there are many addresses that are designated for other special purposes.

Public IP is given to you by your ISP Provider while the private IP is given to you or designated to your computer by your IT in your company

Public addresses are assigned by InterNIC and consist of class-based network IDs or blocks of CIDR-based addresses (called CIDR blocks) that are guaranteed to be globally unique to the Internet.

When the public addresses are assigned, routes are programmed into the routers of the Internet so that traffic to the assigned public addresses can reach their locations. Traffic to destination public addresses are reachable on the Internet.

For example, when an organization is assigned a CIDR block in the form of a network ID and subnet mask, that [network ID, subnet mask] pair also exists as a route in the routers of the Internet. IP packets destined to an address within the CIDR block are routed to the proper destination.

Private Addresses

Each IP node requires an IP address that is globally unique to the IP internetwork. In the case of the Internet, each IP node on a network connected to the Internet requires an IP address that is globally unique to the Internet. As the Internet grew, organizations connecting to the Internet required a public address for each node on their intranets. This requirement placed a huge demand on the pool of available public addresses.

When analyzing the addressing needs of organizations, the designers of the Internet noted that for many organizations, most of the hosts on the organization's intranet did not require direct connectivity to Internet hosts. Those hosts that did require a specific set of Internet services, such as the World Wide Web access and e-mail, typically access the Internet services through Application layer gateways such as proxy servers and e-mail servers. The result is that most organizations only required a small amount of public addresses for those nodes (such as proxies, routers, firewalls, and translators) that were directly connected to the Internet.

For the hosts within the organization that do not require direct access to the Internet, IP addresses that do not duplicate already-assigned public addresses are required. To solve this addressing problem, the Internet designers reserved a portion of the IP address space and named this space the private address space . An IP address in the private address space is never assigned as a public address. IP addresses within the private address space are known as private addresses . Because the public and private address spaces do not overlap, private addresses never duplicate public addresses.

The private address space specified in RFC1918 is defined by the following three address blocks:

• 10.0.0.0/8 The10.0.0.0/8 private network is a class A network ID that allows the following range of valid IP addresses:10.0.0.1 to10.255.255.254. The10.0.0.0/8 private network has24 host bits that can be used for any subnetting scheme within the private organization.

• 172.16.0.0/12 The172.16.0.0/12 private network can be interpreted either as a block of16 class B network IDs or as a20-bit assignable address space (20 host bits) that can be used for any subnetting scheme within the private organization. The172.16.0.0/12 private network allows the following range of valid IP addresses:172.16.0.1 to172.31.255.254.

• 192.168.0.0/16 The192.168.0.0/16 private network can be interpreted either as a block of256 class C network IDs or as a16-bit assignable address space (16 host bits) that can be used for any subnetting scheme within the private organization. The192.168.0.0/16 private network allows the following range of valid IP addresses:192.168.0.1 to192.168.255.254.