> For the complete documentation index, see [llms.txt](https://isubasinghe.gitbook.io/isithas-wiki/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://isubasinghe.gitbook.io/isithas-wiki/computer_science/distributed_systems/computer_networks.md).

# Computer Networks

## Basic idea

Spatially separated computers exchanging messages over a shared physical medium according to a layered stack of protocols (link → IP → TCP/UDP → application).

## Key facts

* Layered model: physical / link / network (IP) / transport (TCP, UDP) / application.
* Internet: a network of networks linked by ISPs and backbone routers; addresses via IP.
* Intranet: privately administered subnet of the Internet, isolated by firewalls.
* Wireless paradigms enable mobile, ubiquitous and IoT computing.

Common distributed systems based on widely used computer networks

* The internet
* Intranets
* Wireless networks

## The internet

Consists of a large number of interconnected collection of computer networks of different types. Features include:

* Computers interacting using a common means of communicating.
* Many different services (applications) (WWW, FTP, SFTP)
* A number of Intranets linked by backbones
* Internet Service Providers (ISPs), that provide access to services on the Internet while providing local services such as email and web hosting.
* A backbone network link with high transmission capacity
* Communication via Satellite, fiber optic cables and other high bandwidth circuits.

![Internet](/files/y1gMGqPkmGn7gFWHywAR)

## Intranets

Is a portion of the Internet that is seperately administrated by organisations. Features:

* A boundary that can be configured to enforce local security policies.
* Several local area connections (LANs) linked by backbone connections.
* A connection to the Internet via a router allowing users within the intranet to acces services on the Internet.
* Firewalls to protect an intranet by preventing unauthorized messages leaving or entering by filtering incoming and outgoing messages.

![Intranet](/files/k3tPldm4N4fPc6E1jkKD)

## Wireless networks

Wireless networks allow the intergration of small computing devices (laptops, mobile phones etc)

Three popular paradigms that use wireless networks are:

1. **Mobile computing :** The mobile computing paradigm allows users to perform computing tasks on the move, while being part of a distributed system.
2. **Ubiquitous computing :** Where cheap computing devices are embedded in devices that are used in parts of parts of a distributed system.
3. **Internet of Things :** Everyday objects are addressable and connected to the internet and may be part of a distributed system.


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