Siemens SINAUT MD741-1 System Manual page 132

Glossary
IP packet
IPsec
132
distinction is made between networks of Class A, B and C - the two
address components may be of different sizes:
1st byte
Class A Netw. addr.
Class B Netw. addr.
Class C
It can be seen from the first byte of the IP address whether the IP
address designates a network of Class A, B or C. The following
definitions apply:
Value of the
1st byte
Class A 1-126
Class B 128-191
Class C 192-223
If you do the arithmetic, you can see that there can be a maximum of
126 Class A networks worldwide, and each of these networks can
comprise a maximum of 256 x 256 x 256 hosts (3 bytes of address
space). There can be 64 x 256 Class B networks, each of which can
contain up to 65,536 hosts (2 bytes of address space: 256 x 256).
There can be 32 x 256 x 256 Class C networks, each of which can
contain up to 256 hosts (1 byte of address space).
See Datagram
IP security (IPsec) is a standard that makes it possible to ensure the
authenticity of the sender, the confidentiality and the integrity of the
data in IP datagrams by means of encryption. The components of
IPSec are the Authentication Header (AH), the Encapsulating Security
Payload (ESP), the Security Association (SA), the Security Parameter
Index (SPI) and the Internet Key Exchange (IKE).
When communication starts the computers involved clarify the method
used and its implications, e.g. Transport Mode or Tunnel Mode.
In Transport Mode an IPSec header is inserted into each IP datagram
between the IP header and the TCP or UDP header. As the IP header
is not changed this mode is suitable only for a host-to-host connection.
In Tunnel Mode an IPSec header and a new IP header are inserted in
front of the entire IP datagram. This means that the original datagram
is contained, encrypted as a whole, in the payload of the new
datagram.
The Tunnel Mode is used in the VPN: the devices at the tunnel ends
perform the encryption and decryption of the datagrams, while the
datagrams themselves remain completely protected as they pass
through the tunnel, i.e. during transmission via a public network.
2nd byte 3rd byte
Host addr.
Host addr.
Netw. addr.
Bytes for the Bytes for the host
network address
address
1
2
3
4th byte
Host addr.
3
2
1
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