Networks

A network is a collection of computers and other devices that can send data and receive data from one another, more or less in real timer.

Node: any device in the network. Host: A node that is general-purpose computer. A protocol is a precise set of rules defining how computers communicate: the format of addresses, how data is split into packets, and so on. IP: Internet Protocol IP's two protocols are:

• IPv4: Which uses 32-bit addresses.
• IPv6: Which uses 128-bit addresses and adds a few other technical features to assist with routing.

In both, IPv4 and IPv6, data is sent across the internet layer in packets called datagrams

The layers of a Network

• The Host-to-network layer: Defines how a particular network interface sends IP datagrams over its physical connection to the local network and the world.

• The Internet layer: Defines how bits and bytes of data are organized into the larger groups called packets, and addressing scheme by which different machines find one another.

• The Transport layer: Is responsible for ensuring that packets are received in the order they were sent and that no data is lost or corrupted. There are two primary protocols at this level:

  • TCP: The Transmission Control Protocol is a high-overhead protocol that allows for retransmission of lost or corrupted data and delivery of bytes in the order they were sent.
  • UDP:The User Datagram Protocol allows the receiver to detect corrupted packets but does not guarantee that packets are delivered in the correct order (or at all).

• The Application Layer: Is the layer that delivers data to the user.

---

The Internet

Internet is the world's largest IP-based network. Each computer on the internet has at least one IP address by wich can be identifyed. Other IP networks are called:

• internet: (lowercase i) A High-Security internal network that is not connected to the global Internet.
• Intranet: Corporate practices of putting lots of data on internal web servers that are not visible to users outside the local network.

Internet Address Blocks

To avoid the conflict of two devices having the same IP address, blocks of IPv4 are assigned to Internet service providers (ISPs) by their regional Internet registry.

Network Address Translation

The routers that connect the local networks to the ISP translate these local addresses to a much smaller set of routable addresses. NAT consists in converting, in real time, the addresses used in the transported packets.

Firewalls

The hardware and software that sit between the Internet and the local network, chacking all the data that comes in or out to make sure it's kosher, is called a firewall. The firewall, which often is part of the router, may perform other tasks, such as Network Address Translation. Firewalls usually operate at the level of the transport or internet layer.

Proxy Servers

Proxy Servers are related to firewalls. Proxy server can act as a go-between within connections to the outside world. Proxies can also be used for FTP services and other connections. One of the security advantages of using a proxy server is that external hosts only find out about the proxy server. They do not learn the names and IP addresses of the internal machines, making it more difficult to hack into internal systems. Proxy servers can also be used to implement local caching. Proxy servers normally operate at the application layer. SOCKS proxy servers can operate at transport layer.

The Client/Server Model

A client/server application typically stores large quantities of data on an expensive, high-powered server or cloud of servers while most of the program logic and the user interface is handled by client software running on relatively cheap personal computers. In some cases, the same program may be both a client and a server.