TCP/IP and IPX routing Tutorial

Introduction

This tutorial is intended to supply enough information to set up a relatively simple WAN-connected internetwork, or Internet-connected LAN. Explanations of IP addresses, classes, netmasks, subnetting, and routing are provided, and several example networks are considered.

A basic explanation of IPX routing is also included.

All brand names and product names are trademarks of their respective companies.

The IP Address and Classes

Hosts and networks

IP addressing is based on the concept of hosts and networks. A host is essentially anything on the network that is capable of receiving and transmitting IP packets on the network, such as a workstation or a router. It is not to be confused with a server: servers and client workstations are all IP hosts.

The hosts are connected together by one or more networks. The IP address of any host consists of its network address plus its own host address on the network. IP addressing, unlike, say, IPX addressing, uses one address containing both network and host address.

How much of the address is used for the network portion and how much for the host portion varies from network to network.

IP addressing

An IP address is 32 bits wide, and as discussed, it is composed of two parts: the network number, and the host number [1, 2, 3]. By convention, it is expressed as four decimal numbers separated by periods, such as "200.1.2.3" representing the decimal value of each of the four bytes. Valid addresses thus range from 0.0.0.0 to 255.255.255.255, a total of about 4.3 billion addresses. The first few bits of the address indicate the Class that the address belongs to:


Class         Prefix     Network Number    Host Number

  A             0         Bits 1-7          Bits 8-31
  B    	        10        Bits 2-15         Bits 16-31
  C             110       Bits 3-24         Bits 25-31
  D             1110      N/A
  E             1111      N/A

The bits are labelled in network order, so that the first bit is bit 0 and the last is bit 31, reading from left to right. Class D addresses are multicast, and Class E are reserved. The range of network numbers and host numbers may then be derived:

Class     Range of Net Numbers          Range of Host Numbers

   A         0 to 126                      0.0.1 to 255.255.254
   B         128.0 to 191.255              0.1 to 255.254
   C         192.0.0 to 254.255.255        1 to 254

Any address starting with 127 is a loopback address and should never be used for addressing outside the host. A host number of all binary 1's indicates a directed broadcast over the specific network. For example, 200.1.2.255 would indicate a broadcast over the 200.1.2 network. If the host number is 0, it indicates "this host". If the network number is 0, it indicates "this network" [2].

All the reserved bits and reserved addresses severely reduce the available IP addresses from the 4.3 billion theoretical maximum. Most users connected to the Internet will be assigned addresses within Class C, as space is becoming very limited. This is the primary reason for the development of IPv6, which will have 128 bits of address space.

Basic IP Routing

Classed IP Addressing and the Use of ARP

Consider a small internal TCP/IP network consisting of one Ethernet segment and three nodes. The IP network number of this Ethernet segment is 200.1.2. The host numbers for A, B, and C are 1, 2, and 3 respectively. These are Class C addresses, and therefore allow for up to 254 nodes on this network segment.

Each of these nodes have corresponding Ethernet addresses, which are six bytes long. They are normally written in hexadecimal form separated by dashes (02-FE-87-4A-8C-A9 for example).

In the diagram above and subsequent diagrams, we have emphasized the network number portion of the IP address.

Suppose that A wanted to send a packet to C for the first time, and that it knows C's IP address. To send this packet over Ethernet, A would need to know C's Ethernet address. The Address Resolution Protocol (ARP) is used for the dynamic discovery of these addresses [1].

ARP keeps an internal table of IP address and corresponding Ethernet address. When A attempts to send the IP packet destined to C, the ARP module does a lookup in its table on C's IP address and will discover no entry. ARP will then broadcast a special request packet over the Ethernet segment, which all nodes will receive. If the receiving node has the specified IP address, which in this case is C, it will return its Ethernet address in a reply packet back to A. Once A receives this reply packet, it updates its table and uses the Ethernet address to direct A's packet to C. ARP table entries may be stored statically in some cases, or it keeps entries in its table until they are "stale" in which case they are flushed.

Consider now two separate Ethernet networks that are joined by an IP router, C.

Device C is acting as a router between these two networks. A router is a device that choses different paths for the network packets, based on the addressing of the IP frame is is handling. Different routes connect to different networks. The router will have more than one address as each route is part of a different network.

Since there are two separate Ethernet segments, each network has its own Class C network number. This is necessary because the router must know which network interface to use to reach a specific node, and each interface is assigned a network number. If A wants to send a packet to E, it must first send it to C who can then forward the packet to E. This is accomplished by having A use C's Ethernet address, but E's IP address. C will receive a packet destined to E and will then forward it using E's Ethernet address. These Ethernet addresses are obtained using ARP as described earlier.

If E was assigned the same network number as A, 200.1.2, A would then try to reach E in the same way it reached C in the previous example - by sending an ARP request and hoping for a reply. However, because E is on a different physical wire, it will never see the ARP request and so the packet cannot be delivered. By specifying that E is on a different network, the IP module in A will know that E cannot be reached without having it forwarded by some node on the same network as A.

Direct vs. Indirect Routing

Direct routing was observed in the first example when A communicated with C. It is also used in the last example for A to communicate with B. If the packet does not need to be forwarded, i.e. both the source and destination addresses have the same network number, direct routing is used.

Indirect routing is used when the network numbers of the source and destination do not match. This is the case where the packet must be forwarded by a node that knows how to reach the destination.

In the last example, A wanted to send a packet to E. For A to know how to reach E, it must be given routing information that tells it who to send the packet to in order to reach E. This special node is the "gateway" or router between the two networks. A Unix-style method for adding a routing entry to A is

route add [destination_ip] [gateway] [metric]

Where the metric value is the number of hops to the destination. In this case,

route add 200.1.3.2 200.1.2.3 1

will tell A to use C as the gateway to reach E. Similarly, for E to reach A,

route add 200.1.2.1 200.1.3.10 1

will be used to tell E to use C as the gateway to reach A.

It is necessary that C have two IP addresses - one for each network interface. This way, A knows from C's IP address that it is on its own network, and similarly for E. Within C, the routing module will know from the network number of each interface which one to use for forwarding IP packets.

In most cases it will not be necessary to manually add this routing entry. It would normally be sufficient to set up C as the default gateway for all other nodes on both networks. The default gateway is the IP address of the machine to send all packets to that are not destined to a node on the directly-connected network. The routing table in the default gateway will be set up to forward the packets properly, which will be discussed in detail later.

Static vs. Dynamic Routing

Static routing is performed using a preconfigured routing table which remains in effect indefinitely, unless it is changed manually by the user. This is the most basic form of routing, and it usually requires that all machines have statically configured addresses, and definitely requires that all machines remain on their respective networks. Otherwise, the user must manually alter the routing tables on one or more machines to reflect the change in network topology or addressing. Usually at least one static entry exists for the network interface, and is normally created automatically when the interface is configured.

Dynamic routing uses special routing information protocols to automatically update the routing table with routes known by peer routers. These protocols are grouped according to whether they are Interior Gateway Protocols (IGPs) or Exterior Gateway Protocols. Interior gateway protocols are used to distribute routing information inside of an Autonomous System (AS). An AS is a set of routers inside the domain administered by one authority. Examples of interior gateway protocols are OSPF and RIP. Exterior gateway protocols are used for inter-AS routing, so that each AS may be aware of how to reach others throughout the Internet. Examples of exterior gateway protocols are EGP and BGP. See RFC 1716 [11] for more information on IP router operations.

Advanced IP Routing

The Netmask

When setting up each node with its IP address, the netmask must also be specified. This mask is used to specify which part of the address is the network number part, and which is the host part. This is accomplished by a logical bitwise-AND between the netmask and the IP address. The result specifies the network number. For Class C, the netmask will always be 255.255.255.0; for Class B, the netmask will always be 255.255.0.0; and so on. When A sent a packet to E in the last example, A knew that E wasn't on its network segment by comparing A's network number 200.1.2 to the value resulting from the bitwise-AND between the netmask 255.255.255.0 and the IP address of E, 200.1.3.2, which is 200.1.3.

The netmask becomes very important, and more complicated, when "classless" addressing is used.

Hierarchical Sub-Allocation of Class C Addresses

To make more efficient use of Class C addresses in the Internet community, these addresses are subnetted hierarchically from the service provider to the organization. They are allocated bitmask-oriented subsets of the provider's address space [4, 5]. These are classless addresses.

Consider the following example of a small organization consisting of two Ethernet segments connecting to an Internet service provider using a WAN router that emulates an additional network segment, such as FPIPE. The service provider has been allocated several different Class C addresses to be used for its clients. This particular organization has been allocated the network number 210.20.30, and the gateway address at the provider end is 210.20.30.254.

Networks 210.20.30.0->63,
         210.20.30.64->127,
         210.20.30.192->255
Netmask  255.255.255.192

We have expanded the last byte of the IP address so that we can show the network subaddressing. The standard IP address nomeclature is shown below this expanded version.

If the organization happened to have just one computer, C, and the entire Class C address is available for use, then the IP address for C may be anything in the range 210.20.30.1 to 210.20.30.253, and its default gateway would be 210.20.30.254 with netmask 255.255.255.0.

However, with two networks plus FPIPE, which must also be on its own network, the Class C address must somehow be subnetted. This is accomplished by using one or more of the bits that are normally allocated to the host number as part of the Class C address, in order to extend the size of the network number. In this case, 210.20.30 has been extended to include four networks, and the netmask has changed to 255.255.255.192 to reflect the additional use of two bits for the network number in the IP address.

Writing the netmask 255.255.255.192 in binary (from FFFFFFC0 in hex) is 11111111/11111111/11111111/11000000, with /' separating the bytes for clarity. Since the organization is allocated all of 210.20.30 (D2141E hex), it has the use of the four following network numbers (in binary):

 Net# IP Network Number

0    11010010/00010100/00011110/00
1    11010010/00010100/00011110/01
2    11010010/00010100/00011110/10
3    11010010/00010100/00011110/11

This leaves 6 bits at the end to use for host number, leaving space for 62 host nodes per network (all 0's and all 1's are reserved). The following addresses are therefore valid for hosts to use:

Net# Address Range

0    210.20.30.1 to 210.20.30.62
1    210.20.30.65 to 210.20.30.126
2    210.20.30.129 to 210.20.30.190
3    210.20.30.193 to 210.20.30.254

In this example, Net#2 is reserved for future use.
The IP addresses and netmasks for each interface are:

Interface      IP Address          Netmask

Node A         210.20.30.1         255.255.255.192
Node B         210.20.30.2         255.255.255.192
Node C (AB)    210.20.30.10        255.255.255.192
Node C (DE)    210.20.30.70        255.255.255.192
Node C (WAN)   210.20.30.200       255.255.255.192
Node E         210.20.30.81        255.255.255.192
Node F         210.20.30.82        255.255.255.192

The routing tables will be set for each node as follows. The destination address 0.0.0.0 indicates the default destination, if no other specific routes are configured for the given packet destination. This default destination is where all packets will be sent, and it is assumed that this destination is capable of forwarding these packets to the ultimate destination, or to another router along the appropriate path.

Node A:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        210.20.30.10   210.20.30.1
210.20.30.0      5.255.255.192     210.20.30.1    210.20.30.1

Node B:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        210.20.30.10   210.20.30.2
210.20.30.0    255.255.255.192     210.20.30.2    210.20.30.2

Node C:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        210.20.30.254  210.20.30.200
210.20.30.0    255.255.255.192     210.20.30.10   210.20.30.10
210.20.30.64   255.255.255.192     210.20.30.70   210.20.30.70
210.20.30.192  255.255.255.192     210.20.30.200  210.20.30.200

Node E:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        210.20.30.70   210.20.30.81
210.20.30.64   255.255.255.192     210.20.30.81   210.20.30.81

Node F:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        210.20.30.70   210.20.30.82
210.20.30.64   255.255.255.192     210.20.30.82   210.20.30.82

Node G:
Network Address     Netmask        Gateway        Interface

210.20.30.0       255.255.255.0  210.20.30.200  210.20.30.254

(Plus all other pertinent entries)

The metric value, or hop count, is optional, but would be 0 for all gateways that are the same as the hosts, and greater than 0 if the destination is reached via one or more gateways. The metric for the default routes are indeterminate, but would always be at least 1.

For example, if D sent an ICMP echo request packet out onto the Internet, let's say to address 140.51.120.30, then first D would AND the netmask 255.255.255.192 with 140.51.120.30 to determine the network number. It would then find that it does not match the network number 210.20.30.64, and so it chooses the default route which points to the gateway 210.20.30.70. It then uses the Ethernet address of Node C (DE) to forward the IP packet to the gateway.

When C receives this packet, it will see that it is destined to 140.51.120.30. It checks all the routes in its table and determines that this address is not located on any of the listed networks in the routing table, and so it chooses the default route. It uses the WAN interface, of IP address 210.20.30.200 to send the packet to 210.20.30.254 (G). From then on, the packet will propagate from gateway to gateway until it reaches 140.51.120.30. When this node replies, the packet will be inbound on interface 210.20.30.200 (C) with destination address 210.20.30.81 (D). Node C will discover that 210.20.30.81 is on the 210.20.30.64 network and uses the interface 210.20.30.70 to send the packet back home to D.

TCP/IP Setup Examples by Protocol Stack and Platform

Two examples will be presented to explain how to set up the IP addressing and routing information when connecting to an Internet service provider using FPIPE. The first case is when only one machine will be connected, and the other case describes the connection of a LAN to the Internet. The third example briefly illustrates the addressing and routing techniques for connecting two LANs over a point-to-point WAN connection.

Example 1: Single Node Connection to WAN Gateway

Assume that the node is assigned the IP address 210.20.30.45, and that the gateway address is 199.99.88.77.

The netmask for A may be set to 255.255.255.255, indicating no other nodes on the local network, and the gateway is set to 199.99.88.77. A default route must be set up at Node A as well, which provides the route for all packets whose destination does not corresponding to any specific routing entries.

Node A:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        199.99.88.77   210.20.30.45

Node G:
Network Address     Netmask        Gateway        Interface

210.20.30.45   255.255.255.255     199.99.88.77   199.99.88.77

(Plus all other pertinent entries)

The routing for Node G is highly dependent on the context, and the above entry only serves as an example. The netmask of all 1's in this case is used to only allow packets destined to 210.20.30.45 to be forwarded to Node A, as there may be 253 other nodes connected in a similar way under this Class C network 199.99.88.0.

When the protocol stack's configuration asks for a default gateway, specifying 199.99.88.77 will cause the default routing entry 0.0.0.0 to be added automatically. It must be added manually if for some reason the stack does not ask for it.

The specific methods of configuring each protocol stack will be explained in detail in Example 2.

Example 2: LAN Connection to WAN Gateway

The following network topology will be used as an example, where one LAN is connected to the Internet for simplicity. This will also demonstrate the use of a different netmask for creating two Class C subnets. Note however that the remote WAN gateway may have an IP address outside the local Class C network, in which case the local WAN gateway interface will usually have an IP address on the same network as the remote WAN gateway. If this is the case, subnetting as shown below may not be necessary, unless more than one local network segment is involved.

Networks 210.20.30.0->127,
         210.20.30.128->255
Netmask  255.255.255.128

Node A is one of the many workstations on the Ethernet segment Net 0. Node Z is the gateway from this Ethernet to the Internet service provider's gateway machine G. Some of the other workstations have been labelled as B to Y for illustration, but will not be referred to in this example as their setup will be the same as for A.

In this case, since only two subnets were needed, only one bit from the host address space need be sacrificed. Writing the netmask 255.255.255.128 in binary (from FFFFFF80 in hex) is 11111111/11111111/11111111/10000000, with /' separating the bytes for clarity. Since the organization is allocated all of 210.20.30 (D2141E hex), it has the use of the two following network numbers (in binary):

Net# IP Network Number

0    11010010/00010100/00011110/0
1    11010010/00010100/00011110/1

This leaves 7 bits at the end to use for host number, leaving space for 126 host nodes per network (all 0's and all 1's are reserved). The following addresses are therefore valid for hosts to use:

Net# Address Range

0    210.20.30.1 to 210.20.30.126
1    210.20.30.129 to 210.20.30.254

The IP addresses and netmasks for each interface are:

Interface           IP Address          Netmask

Node A              210.20.30.1         255.255.255.128
Node Z (Net 0)      210.20.30.126       255.255.255.128
Node Z (Net 1)      210.20.30.200       255.255.255.128

The routing tables will be set for each node as follows. Note that the destination address 0.0.0.0 indicates the default destination, if no other specific routes are indicated.

Node A:
Network Address     Netmask        Gateway        Interface

0.0.0.0             0.0.0.0        210.20.30.126  210.20.30.1
210.20.30.0    255.255.255.128     210.20.30.1    210.20.30.1

Node Z:
Network Address     Netmask        Gateway        Interface

0.0.0.0        0.0.0.0        210.20.30.254  210.20.30.200
210.20.30.0    255.255.255.128     210.20.30.126  210.20.30.126
210.20.30.128  255.255.255.128     210.20.30.200  210.20.30.200

Node G:
Network Address     Netmask        Gateway        Interface

210.20.30.0    255.255.255.0  210.20.30.200  210.20.30.254

(Plus all other pertinent entries)

Example 3: Closed WAN-Connected Internetwork

This is an example of how to connect two LANs together over a point-to-point WAN link. It is assumed that the network is closed, and is therefore not connected to the Internet. There is significant freedom in choosing the IP addresses for this network. However, they should be consistent with the assigned address space reserved by the Internet Assigned Numbers Authority (IANA) for use by private networks [8]:

10.0.0.0    -    10.255.255.255
172.16.0.0  -    172.31.255.255
192.168.0.0 -    192.168.255.255

In this example, the Class B networks 172.20 and 172.21 will be used for each LAN, and the Class C network 192.168.100 will be used for the WAN link.

Networks 172.20.0.0->172.20.255.255 mask 255.255.0.0,
         172.21.0.0->172.21.255.255 mask 255.255.0.0,
         192.168.100.0->192.168.100.255 mask 255.255.255.0

The IP addresses and netmasks for each interface are:

Interface      IP Address          Netmask

Node A              172.20.1.1          255.255.0.0
Node Y (Net 0)      172.20.254.254      255.255.0.0
Node Y (Net 2)      192.168.100.1       255.255.255.0
Node Z (Net 1)      172.21.254.254      255.255.0.0
Node Z (Net 2)      192.168.100.2       255.255.255.0
Node K              172.21.1.1          255.255.0.0

The routing tables will be set for each node as follows. Note that no default routes are listed for routers Y and Z. If Y was Z's default router, and vice versa, routing loops will occur for packets destined to nodes not on either network. It is acceptable for Node A to have a default route to Y, since Y may then discard the packet if the destination is unreachable.

Node A:
Network Address     Netmask        Gateway        Interfacei
0.0.0.0             0.0.0.0        172.20.254.254 172.20.1.1
172.20.0.0          255.255.0.0    172.20.1.1     172.20.1.1

Node Y:
Network Address     Netmask        Gateway        Interface
172.21.0.0          255.255.0.0    192.168.100.2  192.168.100.1
172.20.0.0          255.255.0.0    172.20.254.254 172.20.254.254
192.168.100.0       255.255.255.0  192.168.100.1  192.168.100.1

Node Z:
Network Address     Netmask        Gateway        Interface
172.20.0.0          255.255.0.0    192.168.100.1  192.168.100.2
172.21.0.0          255.255.0.0    172.21.254.254 172.21.254.254
192.168.100.0       255.255.255.0  192.168.100.2  192.168.100.2

Node K:
Network Address     Netmask        Gateway        Interface
0.0.0.0             0.0.0.0        172.21.254.254 172.21.1.1
172.21.0.0          255.255.0.0    172.21.1.1     172.21.1.1

If several point-to-point WAN links are required throughout the internetwork, the YZ Net 2 link may be subnetted to allow for 64 different point-to-point links within the 192.168.100.0 address space. This is done using the netmask 255.255.255.252, dividing the Class C network into 64 subnets with 2 host bits, allowing for 2 actual node addresses and 2 reserved for "this network" and "broadcast".

IPX Routing

The following is a brief introduction to IPX routing in the context of a Novell environment. For more information, consult Novell's IPX Router reference.

Because ipx is always dynamically routed, and the routing architecture works by "learning" network addressing automatically, there is usually no need to do anything special in the setup of an IPX network in order to get routing to function. Thus this section is provided for completeness only.

An IPX address consists of a 4-byte Network Number, a 6-byte Node Number, and a 2-byte Socket Number. The node number is usually the hardware address of the interface card, and must be unique inside the particular IPX network. The network number must be the same for all nodes on a particular physical network segment. Socket numbers correspond to the particular service being accessed. Consider the following IPX network:

Networks 1A2B3C4D and DDEEAADD

Nodes A and D are Novell NetWare workstations, and Nodes B, C and E are Novell NetWare Servers. Node C has two Ethernet cards and acts as an IPX router between the two networks.

The NetWare Servers broadcast routing information and service advertisements to all nodes on the network segment using RIP/SAP or NLSP. Node C forwards this information to connected networks, so that workstations are made aware of the addresses of all file and print servers available, and servers are made aware of the routes to these other servers.

To address a service running on a server, each server has its own Internal Network Number, which is placed in the network number field of the IPX header.

For example, suppose A wants to access the file server E whose internal network number is 5E1C0155. A would have been made aware of E's address through service advertisements broadcasted by C. To learn how to reach E, it broadcasts a routing request. C receives this request and returns its own hardware node number. A therefore addresses an IPX packet to E using E's internal network number of 5E1C0155 and node number 22-5A-4D-8C-C3-DA. The Ethernet header's destination address is Node C's node address of 34-56-78-9A-BC-DE. C then receives this IPX packet and observes that the IPX packet header's destination address is not its own, so it transmits the packet on network DDEEAADD knowing that E is on that network, using an Ethernet header destination address of 22-5A-4D-8C-C3-DA.