This section describes some of the basic concepts of TCP/IP networking.
IP addresses are used to identify hosts or interfaces on IP networks. They are comprised of four sets of numbers or bytes. IP addresses are written in dotted decimal format, such as 192.168.32.22.
An IP address consists of two parts:
The portions of the address that identify the network and host are determined by a combination of the class of the network and the subnet mask.
These are the network classes:
A class A network is identified by a number from 1 to 127 in the first byte, such as 10.1.1.1.
A class B network is identified by a number from 128 to 191 in the first byte, such as 172.16.1.1.
A class C network is identified by a number from 192 to 223 in the first byte, such as 192.168.32.1.
A class A network can have 16,777,216 hosts, while a class B network can have 65,536 hosts, and a class C network can have 256 hosts.
The original Internet addressing scheme made it possible for every host on a network to talk directly with every other host on the same network; other hosts were directly accessible if they used the same network number. In class A and class B networks, where very large numbers of hosts with the same network number are available, this scheme is unrealistic because the physical underlying networks are constrained by bandwidth. Ethernet and Token-Ring networks cannot accommodate thousands or hundreds of thousands of hosts on a single, flat network.
Subnet routing allows network administrators of class A and class B networks to create smaller networks from single class A and B host addresses. These smaller, internal networks are called subnets. Subnet masks inside the class A or class B network are not exposed outside of the class A or B network; all changes to accommodate the additional addresses are handled internally. This simplifies routing information to the network and minimizes the amount of information the network must advertise externally.
Inside the network the administrator determines how to reallocate addresses by choosing how many bits of the host portion of each address are used as a subnet address and how many bits are used as the host address. The administrator uses a subnet mask to divide the existing addresses into network and host portions. The subnet mask identifies how much of the existing address can be used as the network portion. The underlying physical network must also be divided into smaller, physical subnets when using a subnet mask to create subnets.
The following example illustrates how to create several subnets from a class B address. The example also illustrates how traffic is received and how the addresses appear to other hosts and routers on the Internet.
An administrator wants to create two subnets from a class B address. The class B address 172.16.0.0 can be divided by masking the first 24 bits of the 32-bit IP address using the mask 255.255.255.0. The class B network will accept all traffic bound for any IP address beginning with the 16-bit network portion 172.16. Therefore, the administrator can create the networks 172.26.224.0 and 172.16.225.0. Valid addresses on the internal network, such as 172.16.224.42 and 172.16.224.12, can be reached from anywhere on the Internet; final delivery is handled by the individual physical subnets that contain the nodes associated with the addresses.
While subnet masks let you break a large network into several smaller networks, supernet masks let you combine small networks to make a single, larger network. Supernetting is useful if you have a class C network and more than 256 hosts.
A supernet mask works in a manner opposite to that of a subnet mask. Instead of using bits from the host portion of the IP address to modify the network portion, a supernet mask borrows bits from the network portion of the IP address to modify the host portion.
For example, an administrator has two class C networks to link together, 192.168.224.0 and 172.168.225.0. These two network addresses differ in only one bit, the last bit of the third field. The administrator can use a supernet mask to make the network portion of the IP addresses use only the first seven bits of the third field, letting the last bit of the third field be part of the host portion. This supernet mask is 255.255.254.0. Adding this extra bit to the host portion of the IP address means the administrator can now have 512 hosts on a single network.
The class C network numbers must be adjacent when converted to binary values.
A system uses broadcast addresses to send information to all hosts on a local network. Packets addressed to the network's broadcast address are transmitted to every host with the same network number as the broadcast address. Broadcast packets are routinely used by the network to share routing information, send ARP requests, and send status and informational messages.
The current convention for broadcast addresses is to represent the broadcast address as the network portion of the address followed by binary ones in all host portions of the address. In this scheme, the broadcast address for network 172.16 is 172.16.255.255.
If the network contains subnets, the broadcast address is relative to the local subnet. For example, host 172.16.12.1 with a subnet mask of 255.255.255.0 has an IP broadcast address of 172.16.12.255.
Most sites assign names to each system in a network because names are easier to remember than IP addresses. On a small, locally contained network, a host name may be only one word, such as WILLOW. However, on larger networks or on networks connected to the Internet, names are longer to denote their location in the organization and ultimately on the Internet. These longer, more detailed names are called fully qualified host names. An example is WILLOW.YOYODYNE.COM where WILLOW is the individual system name, YOYODYNE identifies the organization to which it belongs, and COM indicates that this organization is involved in commerce.
Physical networks are the cables and the associated wiring components that link computers to one another for network communications. Common physical networks are Ethernet, Token-Ring, FDDI (Fiber Distributed Data Interface), point-to-point links, and telephone lines with modems.
This release of the Cisco TCP/IP Suite Stack for Windows supports Ethernet, Token-Ring, FDDI, and Serial Line (direct or telephone lines with modems) physical networks.
Network interface board manufacturers assign a unique hardware, or physical, address to each interface board they produce. These hardware addresses are burned into the boards at the time of manufacture, and can usually be overridden later by a network administrator, if desired.
A hardware address is composed of six groups, or octets, of numbers, separated by colons, such as 00:DD:A8:13:48:C5. The first three octets identify the manufacturer, while the remaining three octets are unique to the board.
Hardware addresses identify individual interfaces and provide fast and efficient delivery of packets on the physical network.
A network interface card, which is installed in the computer, physically connects the computer to the network. The device driver, which is usually included on a diskette that comes with the interface card, provides the software that lets the computer communicate on the network. Together, the interface card and the device driver are responsible for all hardware details of communicating on the network.
For Windows 95 systems, Cisco TCP/IP Suite uses the network driver installed for the interface card. This is set outside of the Cisco TCP/IP Suite configuration.
For Windows 3.x, choose one of these drivers when configuring Cisco TCP/IP Suite:
Used by Windows for Workgroups, NDIS 3.0 drivers are VxD-based and provide the best performance of the three driver types that Cisco TCP/IP Suite supports.
Used by Microsoft LAN Manager and Banyan Vines, NDIS 2. 0 drivers are TSR-based and should only be used if no other driver is available, or if you are running another network protocol that requires NDIS 2.0 drivers.
Used by Novell NetWare clients, ODI drivers typically consist of an LSL.COM file and a network adapter driver, typically a .COM file, both of which must be running before Cisco TCP/IP Suite starts.
Of the three types of drivers, ODI uses more DOS memory than NDIS 2.0 or NDIS 3.0. NDIS 3.0 uses no conventional memory. In most cases, we recommend ODI over NDIS 2.0.
Cisco TCP/IP Suite is a complete software package that lets computers communicate with one another across a physical network. This section describes how information is delivered from one system residing on a TCP/IP network to another.
When one user communicates with another or with an application on another system, a number of events must occur for the communication to be successful. The following steps present a highly simplified view of the events that occur during successful network communication.
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