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3.5 DHCP (Dynamic Host Configuration Protocol)
DHCP is a protocol that enables network devices to automatically retrieve an IP address from a DHCP server. A
DHCP server knows a pool of IP addresses that he can assign to the network devices that request an IP address.
3.6 Subnet Mask
The subnet mask is a bit mask which defines how much of an IP address belongs to the private subnetwork and
what belongs to the host network. For example the IP address 192.168.1.25 in the subnetwork 255.255.255.0 is
split into the host network address 192.168.1 and subnetwork address 25. In order to enable two network devices
to communicate with each other they have to share the same host network address.
3.7 Network Devices
Networks consist of numerous devices which have to be connected to each other. This happens with the help of a
network device. There are different types of network devices which will be described in the following.
3.7.1 Hub
A hub is a device that combines all the connected network devices into one network segment. In other words
packets that are transmitted through a hub will be forwarded to every connected network device and not just to
the device that is to be addressed. Nowadays hubs have mainly been replaced by switches.
3.7.2 Switch
A switch is an intelligent hub that remembers the MAC addresses of the connected interfaces and can therefore
forward packets with a certain destination to that destination without having to send them to all connected in-
terfaces.
3.7.3 Access Point
A wireless access point is a device that connects WLAN adapters in a wireless network. Often an access point is
connected to a wired network and provides access to this network to the WLAN adapters.
3.7.4 Router
While hubs and switches connect devices to local networks, routers connect different networks. In other words
routers are devices that make a network seem like a single IP address to other networks. This is done through a
process called NAT.
3.8 NAT (Network Address Translation)
Network Address Translation was introduced to overcome a shortage of IP addresses. It is a technique which re-
writes source and/or destination IP addresses in packets when they pass through a router or a firewall. This feature
is mostly used to allow multiple hosts in a private network to access the internet.
Normally every host that wants to access the internet has to have a registered IP address. Unfortunately IP adresses
are of limited quantity. NAT makes it possible to have multiple hosts in a private network (for example in the
private IP range 192.168.x.x or 10.x.x.x) which are connected to a router which is the interface to the internet.
The router modifies the packets in a way that the remote host that should be accessed only sees the external IP
address of the router. Answers are sent to the router which modifies the packets and relays them to the host in the
private network that sent the actual request.
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