Appendices
Network Considerations
H
H-11
by merging the chunks back together in the correct order. TCP also assures error-
free communication by use of a checksum within each packet. Finally, TCP uses
the Port ID to specify the specific application within each system that is sending
and receiving the data.
The IP portion of the TCP/IP protocol simply addresses, sends, and receives
packets. It uses the IP address, the Subnet Mask, and the Default Gateway
information. The IP address is described in IP Addresses and Segmented Networks
in this appendix.
The subnet mask is used to separate the network ID from the host ID in an IP
address. For example: if the IP address of a 2680 Series address was
198.178.246.10, and a class C network ID was active, the subnet mask would be
255.255.255.0. When this mask is applied to the address, the network ID is
extracted as 198.178.246 and the host ID as 10.
The default gateway information is only used when your network contains more
than one subnet, or is connected directly to the Internet. When the network ID of
the source and destination address of a packet are different (hosts on different
network segments), the packet is forwarded to the default gateway for delivery.
The default gateway has knowledge of the network IDs of the other network
segments, so it forwards the packet to other gateways on the network until the
packet is delivered to the gateway attached to the destination segment with the
matching network ID. This process is generally referred to as
routing the packet,
and devices that do this are also referred to as
Routers
or
Gateways
.
In many modern networks, the use of subnet masking and the default gateway is
not necessary. These networks contain routers and gateways that support the Proxy
ARP protocol. In this protocol, the routing path between any two hosts is
automatically established by the routers during the ARP process. As a result, the
user doesn’t need to manually enter the subnet mask and default gateway
information.
When the TCP/IP protocol stack software starts operation, it communicates with
an NDIS or ODI driver in a process called
Binding
. During this process, the stack
tells the driver which protocol it is handling. In this way, a driver can direct
packets to more than one protocol stack. For example, it is quite common to have
the Novell IPX/SPX protocol stack and the TCP/IP protocol stack operating over
the same ODI multi-protocol driver.
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