Information

Physical Layer

Lowest level
Where data transfers via electrical currents, voltage, radio frequency, and modulation takes place
A simple problem at this layer could be an Ethernet cable being severed or unplugged from a network switch

Handles error-correction with data packets received from the physical layer
Handles transferring data packets received from the physical layer, to network interfaces via MAC addresses

Routing occurs in this layer, where packets need to be transferred to their destination
Packets may have to travel between many different network switches in large-scale networks (like the internet) depending on the destination's physical location
Packets hopping to each router has the TTL header subtracted from

Various protocols (commonly TCP and UDP) are specified and used here
TCP is used here and operates on a response-based system where packets received need to be acknowledged
UDP is used here and does not need packets to be acknowledged
TCP is good for reliability, whereas UDP is good for speed and lower latency
TCP is typically used for web server and file transfer connections
UDP is typically used for game server and VPN connections

Identifier assigned to the network interface of a device
All network interfaces have an unique MAC address
A MAC address does not change depending on where the network interface is connected
Composed of hexadecimal digits (0-9, A-F, and a-f), normally in groups of two separated by - or :
Exists at data link layer

Address assigned to a device on a network
Addresses can be assigned dynamically (DHCP) or statically based on the network the interface is connected to
IP address can change depending on the network the network interface is connected to
IPv4 is the most common version at 32-bits of data, and represented in dot-decimal notation
The total number of possible IPv4 addresses are exhausted at large-scale
IPv6 is gaining popularity, is 128-bits of data, and contains a significantly larger pool of available addresses than IPv4

Defined in the Transport layer
Is an endpoint that defines what a service is that is sending or receiving packets
Popular services have well-known port assignments, such as 80 for HTTP, 443 for HTTPS, 21 for FTP, and 6112 for various game servers
Typically referred to in firewall applications, and can be used to block services on a specific device, or network-wide

A header in an IP packet that exists at the Network layer
Defines how many hops a packet is allowed to perform
When a packet transfers from a network switch to another network, the TTL header is subtracted from by one
Used to prevent mis-configured packets from endlessly looping between devices

Data is encapsulated at various points in the network layer
The data that exists in a TCP or UDP (or any protocol at the network layer) packet is encapsulated as the data payload in the IP datagram
The headers specific to an IP datagram, and the data payload that exists in it are then encapsulated as the payload for an Ethernet frame