NAT_S615
Entry ID: 109744660, V1.1, 08/2017
Siemens AG All rights reserved
3 Valuable Information
3.1 General principles
3.1.1 Classless Inter-Domain Routing (CIDR)
Description
The firewall and NAT configuration in the S615 largely use CIDR suffix notation.
CIDR is a method that combines multiple IPv4 addresses into a single address
range by representing an IPv4 address combined with its subnet mask. To this
end, the "/x” suffix indicating the number ("x”) of network mask bits set to "1” is
added to the IPv4 address.
CIDR notation allows the user to reduce routing tables and make better use of the
available address ranges.
Example
IPv4 address 192.168.2.3 with subnet mask 255.255.255.0.
In the binary representation, the network portion of the address comprises three
times 8 bits, i.e. 24 bits. This results in CIDR notation 192.168.2.0/24.
If you want to access all addresses, use the following notation: 0.0.0.0/0.
If you want to access only one address from the network (subnet mask:
255.255.255.255), this results in the following notation: 192.168.2.3/32.
3.1.2 Connection directions in the network
What is decisive for the configuration of the firewall and NAT is the direction of
connection establishment. Therefore, the direction must be defined in advance. A
connection is always actively established by one node. The partner waits passively
for the incoming connection. This defines the destination port (e.g., http on port 80)
of connection establishment.
Normally, the source port of connection establishment is dynamically managed by
the operating system and not known in advance. Exceptions are, for example,
native TCP/UDP connections between S7 CPUs or CPs where a fixed source port
was defined.
S7 connections always have the destination port TCP 102 and a dynamic source
port.
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