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A firewall decides which network traffic is allowed to pass through and which traffic is deemed dangerous.Ports are typically assigned specific purposes, so certain protocols and IP addresses using uncommon ports or disabled ports can be a concern.These regulate inbound and outbound network traffic, separating external public networks--like the global internet--from internal networks like home Wi-Fi networks, enterprise intranets, or national intranets.The basic subnet segments are as follows:

External public networks typically refer to the public/global internet or various extranets.The two most common segment models are the screened host firewall and the screened subnet firewall:

Screened host firewalls use a single screening router between the external and internal networks.As a secured buffer between internal and external networks, these can also be used to house any external-facing services provided by the internal network (i.e., servers for web, mail, FTP, VoIP, etc.).Screened subnet firewalls use two screening routers-- one known as an access router between the external and perimeter network, and another known as the choke router between the perimeter and internal network.Among the networking protocols that hosts use to 'talk' with each other, TCP/IP protocols are primarily used to communicate across the internet and within intranet/sub-networks.By using these identifiers, a firewall can decide if a data packet attempting a connection is to be discarded--silently or with an error reply to the sender--or forwarded.Perimeter networks detail border networks made of bastion hosts -- computer hosts dedicated with hardened security that are ready to endure an external attack.Host firewalls can also dive deeper into web traffic, filtering based on HTTP and other networking protocols, allowing the management of what content arrives at your machine, rather than just where it comes from.Host firewalls or 'software firewalls' involve the use of firewalls on individual user devices and other private network endpoints as a barrier between devices within the network.A network firewall requires configuration against a broad scope of connections, whereas a host firewall can be tailored to fit each machine's needs.Filtering traffic via a firewall makes use of pre-set or dynamically learned rules for allowing and denying attempted connections.Screening routers are specialized gateway computers placed on a network to segment it. They are known as house firewalls on the network-level.Network firewalls may come in the form of any of the following appliance types: dedicated hardware, software, and virtual.

A firewall decides which network traffic is allowed to pass through and which traffic is deemed dangerous. Essentially, it works by filtering out the good from the bad, or the trusted from the untrusted. However, before we go into detail, it helps to understand the structure of web-based networks.

Firewalls are intended to secure private networks and the endpoint devices within them, known as network hosts. Network hosts are devices that ‘talk’ with other hosts on the network. They send and receive between internal networks, as well as outbound and inbound between external networks.

Computers and other endpoint devices use networks to access the internet and each other. However, the internet is segmented into sub-networks or 'subnets' for security and privacy. The basic subnet segments are as follows:

External public networks typically refer to the public/global internet or various extranets.
Internal private network defines a home network, corporate intranets, and other ‘closed’ networks.
Perimeter networks detail border networks made of bastion hosts — computer hosts dedicated with hardened security that are ready to endure an external attack. As a secured buffer between internal and external networks, these can also be used to house any external-facing services provided by the internal network (i.e., servers for web, mail, FTP, VoIP, etc.). These are more secure than external networks but less secure than internal. These are not always present in simpler networks like home networks but may often be used in organizational or national intranets.
Screening routers are specialized gateway computers placed on a network to segment it. They are known as house firewalls on the network-level. The two most common segment models are the screened host firewall and the screened subnet firewall:

Screened host firewalls use a single screening router between the external and internal networks. These networks are the two subnets of this model.
Screened subnet firewalls use two screening routers— one known as an access router between the external and perimeter network, and another known as the choke router between the perimeter and internal network. This creates three subnets, respectively.
Both the network perimeter and host machines themselves can house a firewall. To do this, it is placed between a single computer and its connection to a private network.

Network firewalls involve the application of one or more firewalls between external networks and internal private networks. These regulate inbound and outbound network traffic, separating external public networks—like the global internet—from internal networks like home Wi-Fi networks, enterprise intranets, or national intranets. Network firewalls may come in the form of any of the following appliance types: dedicated hardware, software, and virtual.
Host firewalls or 'software firewalls' involve the use of firewalls on individual user devices and other private network endpoints as a barrier between devices within the network. These devices, or hosts, receive customized regulation of traffic to and from specific computer applications. Host firewalls may run on local devices as an operating system service or an endpoint security application. Host firewalls can also dive deeper into web traffic, filtering based on HTTP and other networking protocols, allowing the management of what content arrives at your machine, rather than just where it comes from.
A network firewall requires configuration against a broad scope of connections, whereas a host firewall can be tailored to fit each machine's needs. However, host firewalls require more effort to customize, meaning that network-based are ideal for a sweeping control solution. But the use of both firewalls in both locations simultaneously is ideal for a multi-layer security system.

Filtering traffic via a firewall makes use of pre-set or dynamically learned rules for allowing and denying attempted connections. These rules are how a firewall regulates the flow of web traffic through your private network and private computer devices. Regardless of type, all firewalls may filter by some combination of the following:

Source: Where an attempted connection is being made from.
Destination: Where an attempted connection is intended to go.
Contents: What an attempted connection is trying to send.
Packet protocols: What ‘language’ an attempted connection is speaking to carry its message. Among the networking protocols that hosts use to ‘talk’ with each other, TCP/IP protocols are primarily used to communicate across the internet and within intranet/sub-networks.
Application protocols: Common protocols include HTTP, Telnet, FTP, DNS, and SSH.
Source and destination are communicated by internet protocol (IP) addresses and ports. IP addresses are unique device names for each host. Ports are a sub-level of any given source and destination host device, similar to office rooms within a larger building. Ports are typically assigned specific purposes, so certain protocols and IP addresses using uncommon ports or disabled ports can be a concern.

By using these identifiers, a firewall can decide if a data packet attempting a connection is to be discarded—silently or with an error reply to the sender—or forwarded.