Switches

Switches are Multi-Input Multi-Output (MIMO) devices that enable
packets to travel from one node to another. Switches connect hosts that
belong to the same layer-2 network. Switches enable forwarding of the
packet received on one port (input) to another port (output) so that
they reach the desired destination node. Switches operate at layer-2 in
the networking model. They forward the traffic based on the destination
Ethernet address in the packet header.

Routers

Routers are special devices that enable packets to travel from one
layer-3 network to another. Routers enable communication between two
nodes on different layer-3 networks that are not directly connected to
each other. Routers operate at layer-3 in the networking model. They
route the traffic based on the destination IP address in the packet
header.

Firewalls

Firewalls are used to regulate traffic to and from a host or a
network. A firewall can be either a specialized device connecting two
networks or a software-based filtering mechanism implemented on an
operating system. Firewalls are used to restrict traffic to a host based
on the rules defined on the host. They can filter packets based on
several criteria such as source IP address, destination IP address, port
numbers, connection state, and so on. It is primarily used to protect
the hosts from unauthorized access and malicious attacks. Linux-based
operating systems implement firewalls through iptables.

Load balancers

Load balancers can be software-based or hardware-based devices that
allow traffic to evenly be distributed across several servers. By
distributing the traffic across multiple servers, it avoids overload of
a single server thereby preventing a single point of failure in the
product. This further improves the performance, network throughput, and
response time of the servers. Load balancers are typically used in a
3-tier architecture. In this model, a load balancer receives a request
from the front-end web server, which then forwards the request to one of
the available back-end database servers for processing. The response
from the database server is passed back to the web server for further
processing.