Software Layers

Software layers correspond to the environment or scope in which their services operate. At the top is the application layer, home to application-specific networking methods used by software applications. For example, a web browser program uses a client-server application model and a specific protocol for interaction between servers and clients, while many file-sharing systems use a peer-to-peer model.

Below this upper layer, the transport layer connects applications on different hosts through a logical channel with appropriate data exchange methods over the network. It provides several services, including routing, reliable delivery (TCP), and unreliable datagram service (UDP).

Networking technologies based on these layers interconnect networks at their boundaries and exchange traffic across them. 

The Internet layer implements Internet Protocol, which enables computers to identify and locate each other by Internet Protocol (IP) addresses and to route their traffic through intermediate (transport) networks. Internet Protocol layer code is physically independent of the type of network it is running on.

At the bottom of the architecture is the link layer, which provides logical connectivity between hosts. Link-layer code is usually the only piece of software that is customized to the physical networking link protocol type. Multiple link layers are implemented and each operates through a type of network links, such as a local area network (LAN) or a wide area network (e.g. Wi-Fi or Ethernet or dial-up connection, ATM, etc.).

Internet Protocol:

A key component of the Internet model is the Internet Protocol (IP). IP enables the Internet to work and essentially, establishes the Internet. Two versions of Internet Protocol exist, IPv4 and IPv6.

IP Addresses:

A DNS resolver connects three name servers to resolve the user-visible domain name “www.wikipedia.org” to determine the IPv4 address 207.142.131.234.

To locate individual computers on a network, the Internet provides IP addresses. IP addresses are used by the Internet infrastructure to direct Internet packets to their destinations. They consist of fixed-length numbers that appear in the packet. IP addresses are usually automatically assigned or configured to devices through DHCP.

However, the network also supports other addressing systems. Users often enter domain names (e.g. “En.wikipedia.org”) instead of IP addresses because they are easier to remember, and the Domain Name System (DNS) converts them to IP addresses, which are more efficient for routing purposes.

IPv4:

Internet Protocol Version 4 (IPv4) defines an IP address as a 32-bit number. Internet Protocol Version 4 (IPv4) was the initial version used in the first generation of the Internet and is still in strong use. It is designed to address ≈4.3 billion hosts. However, the explosive growth of the Internet has led to IPv4 address exhaustion, which entered its final phase in 2011, when the global IPv4 address allocation pool ran out.