Introduction To Modbus TCP/IP
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Fragmentation Offset (FO)
– A byte count from the start of the original
packet sent and set by any router that performs IP router fragmentation.
Time-to-Live (TTL)
– An 8-bit value that is used to limit the number of
routers through which a packet may travel before reaching its destination
(the number of hops/links which the packet may be routed over). This
number is decremented by 1 by most routers and is used to prevent
accidental routing loops. If the TTL drops to zero, the packet is discarded
by either the server that has last decremented it, or the next server that
receives it.
Protocol
– An 8-bit value that is used to allow the networking layer to know
what kind of transport layer protocol is in the data segment of the IP packet.
For example, 1=ICMP, 2=IGMP, 6=TCP, 17=UDP.
Header Checksum
– A 16-bit checksum (1’s complement value) for the
header data that allows a packet to offer verification data to help ensure that
the IP header is valid. This checksum is originally inserted by the sender
and then updated whenever the packet header is modified by a router. This
is used to detect processing errors on the part of the router or bridge where
the packet is not already protected by a link layer Cyclic Redundancy Check
(CRC). Packets with an invalid checksum are discarded by all nodes in an
IP network.
Source IP Address (32 bits)
– The IP address of the source machine
sending the data onto the network. This address is commonly represented
by 4 octets representing decimal values and separated by periods
(255.255.255.10 for example).
Destination IP Address (32 bits)
– The IP address of the destination
machine to which the packet is being routed for delivery. This address is
commonly represented by 4 octets representing decimal values and
separated by periods (255.255.255.10 for example).
Options (Variable Number of Bits/Words)
– These bits are reserved for
special features and are rarely used, but when they are used, the IP header
length will be greater than 5 (five 32-bit words) to indicate the relative size of
the option field.
IP Data (Variable Number of Bits/Words)
- This portion of the packet may
contain any number of nested protocols (TCP, UDP, ICMP, etc.).
The Ethernet Address or MAC Address refers to the Media Access
Control Address that uniquely identifies the hardware of any network device.
This is a unique, 48-bit, fixed address assigned and hard-coded into an
Ethernet device at the factory. This is usually expressed in hexadecimal
form as 12 hex characters (6 bytes), with the first 3 bytes (6 leftmost hex
characters) representing the device manufacturer, and the last 3 bytes (6
rightmost hex characters) uniquely assigned by the manufacturer. All six
bytes taken together uniquely identify the network device.
Do not confuse the Ethernet Address (MAC address) with the Internet
Protocol (IP) Address, which is a 32-bit number assigned
to your computer
(see below) that can change each time you connect to a network.
IP addresses are 32-bit numbers that are administered by an independent
authority (InterNIC) and are unique for any device on the network. The IP
address is a 32-bit value made up of four octets (8 bits), with each octet
having a value between 0-255 (00H-FFH). It is commonly expressed as
four decimal numbers (8-bit values) separated by a decimal point. This
provides about 4.3 billion possible combinations.
IP – Internet Protocol
A Network's Infrastructure
includes the physical hardware
used to transmit data
electronically such as routers,
switches, gateways, bridges,
and hubs.
A router is located at the
gateway where it directs the
flow of network traffic and
determines the route of
packets as they travel from
one network to another
network(s). A router can be
either a hardware device or a
software application.
Ethernet (MAC)
Address
TIP: If you want to determine
the Ethernet address of the
NIC card installed in your PC,
at the DOS command prompt,
type WINIPCFG <Enter>
(Windows 98), or IPCONFIG
/ALL <Enter> (Windows XP).
Internet (IP) Address