-: Routers. Now, while switches operate
at layer two of the OSI model
by making their decisions based on MAC addresses,
routers operate at layer three, making their
decisions based on IP addresses.
Routers are used to connect two or more networks
to form an internetwork.
Such as when you connect your small office
home office router, your internal network,
out to the Internet.
It connects your office's network out
to other office's network over the Internet.
Now, routers are devices that make
routing decisions and they do this
by using IP addresses.
These layer three IP addresses are used
to determine what network a particular host is on
and what path the traffic should take
to go across the wide area network
until it reaches its destination network.
Once the traffic reaches the destination network
or the final router that's involved,
that particular router will conduct
ARP broadcast to locate the correct
host on its local network and pass the traffic
to it using its MAC address at the layer two
which is known as that physical address.
In addition to this important routing function,
routers also provide us with some security
functions too. Access control lists or ACLs
can be configured on the router's interface
to control the flow of traffic into
or out of a certain part of the network.
ACLs are an ordered set of rules
that will either permit or deny traffic
based upon certain characteristics,
like its source or destination IP address,
the source or destination port number
associated with it, and the application
or service being run.
Now, in an effort to get past these access control lists,
attackers will often conduct IP spoofing.
If they can spoof the IP, they can trick
the access control list to think
they're on the approved list and let them in
or let them out.
Since routers are on the external interface
for a network, they're commonly a target for attack as well.
And so out of the box, routers tend to be very insecure
and you need to configure them properly for security.
This includes changing things like your default username
and password, changing the default routing tables,
and changing those default IP internal addresses.
To help protect our routers and our internal networks,
we use a lot of other network devices and technologies,
such as firewalls, intrusion prevention systems,
virtual private network connections, content filters,
and access control lists.
By layering all these defenses, we create
a better defense in depth posture.
Now, we're going to cover all of these protective
devices and technologies throughout this course
but for now, it's sufficient to realize
that using these things helps add up our security.
And that helps us to secure our routers
from various attack methods.