GE 90-30 PLC - Manual (Page 103)

C
C-2 TCP/IP Ethernet Communications User’s Manual - August, 1997 GFK-1084B
IP addresses are written as four decimal integers (0-255) separated by periods (called
“dotted-decimal”) where each integer gives the value of one byte of the IP address. For
example, the 32-bit IP address
00000011 00000000 00000000 00000001
is written as
3.0.0.1
One can distinguish the class of an IP address from the first integer in its dotted-decimal
IP address as follows.
ББББББББ
ББББББББ
Range of first integer
ББББ
ББББ
Class
ББББББББ
ББББББББ
0 - 127 ББББ
ББББ
A
ББББББББ
ББББББББ
128 - 191 ББББ
ББББ
B
ББББББББ
ББББББББ
192 - 223
ББББ
ББББ
C
ББББББББ
ББББББББ
224 - 255
ББББ
ББББ
other
Gateways
Gateways (also known as routers) connect individual physical networks into a system of
networks. When a node needs to communicate with a node on another network, a
gateway transfers the data between the two networks.
The following figure shows gateway G connecting Network 1 with Network 2.
a45405
128.1.0.1
Network 1
128.2.0.1
ООО
ООО
ООО
128.2.0.2
ООО
ООО
ООО
128.2.0.3
ООО
ООО
ООО
Network 2
128.1.0.2
A
B C
G
Figure C-2. Connecting Two Networks with a Gateway
When host B with IP address 128.2.0.1 communicates with host C, it knows from C’s IP
address that C is on the same network. In an Ethernet environment, B can then resolve
C’s IP address to a MAC address (via ARP) and communicate with C directly.
When host B communicates with host A, it knows from A’s IP address that A is on
another network (the netids are different). In order to send data to A, B must have the IP
address of the gateway connecting the two networks. In this example, the gateway’s IP
address on Network 2 is 128.2.0.3.