Networking terms - Explained [Part 1]

Network - a network is simply a group of two or more computers or networking-capable devices that have the ability to communicate. Note that a router is not necessary to have a network, though it is necessary if you‟d like to communicate with other networks (such as the internet or an intranet). Networks come in all sizes and levels of complexity.

NIC - a Network Interface Card is a physical piece of hardware that is, essentially, a TCP/IP stack. It performs packet reception and passes the information to the CPU for further processing.

Packet - a packet is a collection of data represented in bits and bytes, usually displayed in hexadecimal. Each protocol has its own structure that it uses to parse and interpret the received data. Packets occur on anything on Layer 3 and up of the OSI model (you will learn about this later) and have both a source and destination IP address, but not necessarily a destination port. Some protocols are not port-specific.

Frame - a frame, much like a packet, is a collection of data. However, a frame is transported only on Layer 2 of the OSI model. It does not use IP addressing but rather MAC addresses. Examples of this are ARP requests/responses and PPP (Point to Point Protocol) VPNs. They are fundamentally identical to the idea of packets.

Header - the packet header is a portion of IP packets that contain information about the destination to which the packet is being sent. This includes the destination IP address.

Checksum - the checksum is a form of data integrity verification. It is the 1‟s complement (sum XOR 111111 111111) sum of the data in binary. The recipient of the packet will perform the same algorithm. If the checksums are different, it may request that a packet be resent. Though UDP does not perform this request, it still has a checksum and will discard.

Broadcast - a frame or packet that is broadcasted is one that is sent to every device on the network or subnet. The broadcast IP address is determined by the Network IP and the subnet mask, but the broadcast MAC address is FF:FF:FF:FF:FF:FF (48 consecutive 1‟s in binary represented in hexadecimal).

Ports:

Physical - a port, in networking, is a physical port into which you plug cables or other devices (such as USB). These include cables for console, serial, ethernet, fiber, coaxial, and auxiliary connections.

Virtual - a virtual port is simply a „slot‟ on the TCP/IP stack. It allows for communication between internal network services and external machines. They are represented in decimal, but are 16 bits (2 bytes) which means there can be a total of 2^16 -1 possible port numbers (65535). Ports are used for both source and destination of packets. Source ports are randomly generated whilst destination ports of (such as those of common services) are predetermined.

Protocol - a protocol is the communication mechanism used by two or more machines to transfer data (represented in bits/bytes). They also may occur on different layers of the OSI model. Higher-layer protocols use some or all of the the preceding layers for means of transportation.

VPN - a Virtual Private Network is a very cloudy subject, especially when defined by people with little to no experience with them. Virtual Private Networks are used by virtually every major corporation to allow its employees to easily and securely access data and files inside their network without exposing it to the internet. A VPN is a securely encrypted tunnel through which data is sent to and from a network-capable device. The packet is then processed on the opposite end for further processing.
For example, if a company employee needs to access, edit, and save/upload files. She cannot access the files directly through the internet as there is a firewall and DMZ preventing her from doing such. She uses a VPN connection to remotely access a machine inside of the network and is able to see the file server and access, download, and upload the required file. VPNs are often used by malicious hackers (and „hackers‟), as well as law-abiding citizens who value privacy, to change their IP address and mask their internet usage.

Proxy - much like VPN, the term „proxy‟ has been skewed on forums and by inexperienced people. A proxy server is a standalone server that will receive data on one port and relay it to the intended recipient. It is not encrypted, so it does not provide the security that a VPN would. Also, in many proxies, only the HTTP headers are altered making it trivially easy for the destination server to find the original IP, so it does not provide the anonymity of a VPN.

Private Network - this really isn‟t a hard concept to get, but many people simply don‟t understand it. A private network is one that is not directly routed to the internet. Devices inside of a private network can communicate with one another, but without a properly configured router, it is impossible for other devices to communicate with devices inside of the network. There are Three sets of private IP addresses.
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.16.31.255
192.168.0.0 - 192.168.255.255
Any IP address outside of those ranges are public IP addresses. Note that, by default, private addresses have masks of 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively. These can be changed (assuming you are not outside the private range).

NAT / PAT - Network Address Translation is something used by IPv4 to allow multiple systems to use a singular IP address (your external IP address). This is not needed in IPv6 as the vast amount of IPv6 addresses will never be depleted. NAT allows the outside interface using a public (internet) IP address to access devices in a private network using a private IP address (as learned earlier). Port Address Translation (AKA Port Forwarding) is used to map a single virtual port of an external interface to another port (it may or may not be the same port; it is by no means required, but is often convenient) on an internal device. Without NAT, machines outside of a network would not be able to initiate connections to those on the inside. Note that this still allows inside devices to initiate connections to outside devices and the router will keep track of the TCP stream to make sure that NAT/PAT is not required to keep connections alive.

Route (verb) - when a device (such as a router or a multilayer switch) „routes‟ a packet, it is not moving it, but copying it from one network to another. This changes the source IP and header information, but keeps the data segment, to that of the NEW Network Interface Card (NIC) in case a response is needed.

Switch (verb) - Like routing, switching involves moving a packet (or, more specifically, frames) from one device‟s interface to another within one broadcast domain. However, a switch does not use an IP address for its destinations like routing does and therefore does not move across networks, but rather inside of its own network. Any wired devices that are not directly connected to one another and have the same IP schematic are undoubtedly connected to a device that switches packets (IE: a switch, hub, or bridge).