Network devices
Hubs
A
hub serves as a central point to which all of the hosts in a network
connect to. It is an OSI layer 1 device. It receives a signal from one
port and sends it out to all other ports. Sometimes it is called a
multiport repeater (photo credit: Wikipedia)
Today,
these devices are considered obsolete and switches are commonly used
instead. Hubs have numerous disadvantages. They are not aware of the
traffic that passes through them. They create only one large collision
domain. A hub typically operates in half duplex. There is also a
security issue with hubs since the traffic is forwarded to all ports
(except the source port), which makes it possible to capture all traffic
on a network with a network sniffer!
Switches
Like
hubs, a switch is used to connect multiple hosts together, but it has
many advantages over a hub. Switch is an OSI Layer 2 device, which means
that it can inspect received traffic and make forwarding decisions.
Each port on a switch is a separate collision domain and can run in a
full duplex mode (photo credit: Wikipedia).
How switches work
Let’s take a look at the following example:
Host
A is trying to communicate with Host B and sends a packet. A packet
arrives at the switch, which looks at the destination MAC address. The
switch then searches that address in its MAC address table. If the MAC
address is found, the switch then forwards the packet only to the port
that connected to the frame’s destination. If the MAC address is not
found, the switch will flood the frame out all other ports. To learn
which MAC address is associated with which port, switches examine the
source MAC addresses of the receiving packet and store that MAC
addresses in their MAC address table.
What is a MAC address table?
A
MAC address table lists which MAC address is connected to which port.
It is used by switches to make forwarding decisions. The table is
populated by examining the source MAC address of the incoming packet. If
the source MAC address of a packet is not present in the table, the
switch adds an entry to it’s MAC address table.
The picture below show how a MAC address table on a switch looks like:
Difference between a switch and a bridge
A
switch is sometimes called a multiport bridge, but there are
differences between these two devices. A bridge usually has fewer ports
than switch. A switch operates faster because it is hardware-based,
which means that it uses chips (ASICs) when making forwarding decisions.
In contrast, a bridge is software based. A switch can also have
multiple spanning-tree instances while a bridge can have only one.
Switches can also have multiple broadcast domains, one per VLAN.
Routers
A
router is a device that routes packets from one network to another. A
router is most commonly an OSI Layer 3 device. Routers divide broadcast
domains and have traffic filtering capabilities.
The picture below shows a typical home router:
How routers work
A
router uses IP addresses to figure out where to send packets. If two
hosts from different networks want to communicate, they will need a
router between them to route packets
For example, check the following scenario:
Host
A and host B are on different networks. If host A wants to communicate
with host B, it will have to send a packet to the router. The router
receives the packet and checks the destination IP address. If the
destination IP address is in the routing table, the router will forward
the packet out the interface associated with that network.
What is a routing table?
A
routing table lists a route for every network that a router can reach.
It can be statically configured (using IOS commands) or dynamically
learned (using a routing protocol). It is used by routers when deciding
where to forward packets.
The picture below shows how a routing table looks like:
The command to display an IP routing table is show ip route.
In the picture above, you can see that this router has two directly
connected subnets. Let’s take a closer look at the first entry in the
routing table:
„C“
means that the route is a directly connected route. The network in
question is 10.0.0.0/8, and the router will forward each packet destined
for that network out interface FastEthernet0/1.
NOTE – in Windows, you can use the netstat –r command to display the routing table of your system.
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