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ment. If you might connect your private network with the
Internet later via a router, the private IP addresses used in
your local network can be maintained. Since within each
IP address range the first IP address is used as network
IP address and the last IP address is used as Broadcast IP
address, in each case two IP addresses have to be taken
off from the "number of possible host addresses" (see ta-
ble 10.1: Private IP address ranges).
Apart from the organization of IP addresses into public
and private address ranges, IP addresses are also divided
into classes (Class: A, B, C, D, E). Within the classes A, B,
and C are also include the private IP of address ranges de-
scribed before. The categorisation from IP addresses is for
the assignment of public IP address ranges of importance
and essentially depends on the size of a local network
(maximum number of hosts in the network), which is to
be connected with the Internet (see table 2: Classes of IP
addresses). IP addresses can fix (statically) or variable (dy-
namically) to be assigned. If IP addresses in a network are
assigned fix, an IP address must be preset manually with
each network element. If IP addresses in a network are
assigned to the attached network elements automatically
(dynamically), a DHCP server (English DHCP becomes;
Dynamic Host Configuration Protocol) is required for the
dispatching of IP addresses. With a DHCP server an IP
address range for the automatic dispatching of IP addres-
ses can be preset. A DHCP server is usually already inte-
grated in a router (DSL router, ISDN router, Modem router,
WLAN router, ...) integrated. If a network element (e.g. an
instrument) is connected by a network cable directly with a
host (PC), the IP addresses cannot be assigned to the ins-
trument and the host (PC) automatically, since no network
with DHCP server is present here. They have to be preset
therefore at the instrument and at the host (PC) manually.
IP addresses are divided by using subnet mask into a
network quota and into a host quota, so similarly e.g. a
telephone number is divided in pre selection (land and local
area network number) and call number (user number).
Subnet mask have the same form as IP addresses. They
are represented with four decimal numbers separated
by points (e.g. 255.255.255.0). As is the case for the IP
adress range
10.0.0.0 –10.255.255.255
172.16.0.0 –172.31.255.255
192.168.0.0 –192.168.255.255
Table 10.1: Private IP adress ranges
class adress range
A 0.0.0.1 - 127.255.255.255
B 128.0.0.1 - 191.255.255.255
C 192.0.0.1 - 223.255.255.255
D 224.0.0.1 - 239.255.255.255
E 240.0.0.1 - 255.255.255.255
Table 10.2: Classes of IP adresses
subnetz mask CIDR way of writing
255.0.0.0 10.0.0.0/8
255.240.0.0 172.16.0.0/12
255.255.0.0 192.168.0.0/16
255.255.255.0 192.168.0.0/24
net quota host quota
8 Bit 24 Bit
16 Bit 16 Bit
24 Bit 8 Bit
addresses here each decimal number represents a binary
number of 8 bits. The separation between network quota
and host quota is determined by the subnet mask within
an IP address (e.g. the IP address 192.168.10.10 by the
subnet mask 255.255.255.0 is divided into a network quota
192.168.10.0 and a host quota of 0.0.0.10). The allocation
takes place via the transformation of the IP address and
the subnet mask in binary form and afterwards a bit by bit
one logical AND operation between IP address and subnet
mask. The result is the network quota of the IP address.
The host quota of the IP address takes place via the bit by
bit logical NAND operation between IP address and subnet
mask. By the variable allocation of IP addresses in network
quota and host quota via subnet masks, one can specify IP
address ranges individually for large and small networks.
Thus one can operate large and small IP networks and
connect if necessary to the Internet via a router. In smaller
local networks the subnet mask 255.255.255.0 is mostly
used. Network quota (the first 3 numbers) and host quota
(the last number) are simple here without much mathe-
matical expenditure to determine and it can with these
subnet mask up to 254 network elements (e.g. measuring
instruments, hosts/PC's...) in a network be operated at the
same time.
Often also a standard gateway is present in a network. In
most local networks is this gateway with the router to the
Internet (DSL router, ISDN router etc.) is identical. Using
this (gateway -) router a connection can be manufactured
with another network. Thus also network elements, which
are not in the same (local) network, can be reached and/
or network elements from the local network are able to ex-
change data with network elements from other networks.
For a network-spreading data exchange the IP address of
the standard gateway must also be preset. In local net-
works, mostly the first IP address within a network for this
(gateway -) router is used. Mostly routers in a local network
to be used as gateway have an IP address with a „1" in the
last place of the IP address (e.g. 192.168.10.1).
number of possible host adresses
2 − 2 = 16.777.214
24
2 − 2 = 1.048.574
20
2 − 2 = 65.534
16
2 − 2 = 254
8
max . number of networks
126
16.384
2.097.151
Reserved for multicast applications
Reserved for special applications
Remote Control
max . number of hosts
16.777.214
65.534
254
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