3Com Router User Manual

							20
CONFIGURING IP ADDRESS
This chapter contains information on the following topics:
■IP Address Overview
■Troubleshooting IP Address Configuration
■Map between WAN Interface IP Address and Link Layer Protocol Address 
IP Address OverviewIP address is a unique 32-bit address assigned to a host connected to Internet. 
Usually it is composed of two parts: network ID and host ID. Its structure enables 
convenient addressing on Internet. IP address is assigned by Network Information 
Center (NIC) of American National Defense Data Network.
For easy IP address management and convenient networking, IP address of 
Internet is divided into five classes. An IP address consists of the following 3 fields:
■Type field (also called type bit), used to distinguish the type of IP address.
■Network ID field (net-id).
■Host ID field (host-id).
Figure 117   Classification of IP address
Address of class D is a multicast address, mainly used by IAB (Internet Architecture 
Board). Address of class E is reserved for future use. At present, IP addresses are 
mostly of class A, class B and class C.
When using IP addresses, it should also be noted that some of them are reserved 
for special uses, and are seldom used.
0    net-id
host-id
1host-id
110
net-idhost-id
1
110Multicast address
11
1
10Reserved for future use
0 1 2 3 4      8
16          24
31
Class A
Class B
Class C
Class D
Class E
net-id
—Network number
host-idHost number
— net-id
0 
						

							318CHAPTER 20: CONFIGURING IP ADDRESS
The IP addresses a user can use are listed in the following table.
Ta b l e 351   IP address classes and ranges
Important features of IP address:
Some IP addresses are not in a hierarchical structure, which is different from the 
structure of telephone number. In other words, these IP addresses cannot reflect 
any geographical information about the host position.
■When a host is connected to two networks at the same time (such as the host 
used as a router), it must have two corresponding IP addresses with different 
net-ids. Such host is called multihomed host.
■According to Internet concept, several LANs connected via transceiver or 
bridges are still in the same network, so these LANs have the same net-id.
■In terms of IP address, all networks which are assigned with net-ids are equal 
(no matter whether it is a small LAN or a big WAN).
Since 1985, only the net-id of IP address is assigned, while the following host-id is 
controlled by the enterprise. The IP address assigned to an enterprise is only a 
network ID: net-id. The specific host Ids, the host-ids for respective hosts, shall be 
assigned by the enterprise independently and uniquely. If there are many 
enterprise hosts widely scattered, the host IDs may be further divided into internal 
sub-nets to facilitate management. Please note that the division of sub-nets is 
Network 
classIP network range Description
A1.0.0.0 ~ 126.0.0.0Network IDs with all the digits being 0 or all the digits being 
1 are reserved for special use.
Host ID with all the digits being 0 indicates that the IP 
address is the network address, and is used for network 
routing.
Host ID with all the digits being 1 indicates the broadcast 
address, i.e. broadcast to all hosts on the network.
Network ID 127 is used for self-loop interface.
B128.1.0.0 ~ 
191.254.0.0Network IDs with all the digits being 0 or all the digits being 
1 are reserved for special use. 
Host ID with all the digits being 0 indicates that the IP 
address is the network address, and is used for network 
routing.
 Host ID with all the digits being 1 indicates the broadcast 
address, i.e. broadcast to all hosts on the network.
C192.0.1.0 ~ 
223.255.254.0Network IDs with all the digits being 0 or all the digits being 
1 are reserved for special use. 
Host ID with all the digits being 0 indicates that the IP 
address is the network address, and is used for network 
routing.
Host ID with all the digits being 1 indicates the broadcast 
address, i.e. broadcast to all hosts on the network.
DNoneAddresses of class D are multicast addresses.
Host ID with all the digits being 1 indicates the broadcast 
address, i.e. broadcast to all hosts on the network.
ENone255.255.255.255 is used as the whole networks broadcast 
address, and the other addresses are reserved for future use. 
						

							IP Address Overview319
completely internal to the enterprise itself, and seen from the outside, the 
enterprise only has one net-id. When an external message enters this enterprise 
network, the internal router can route according to the sub-net number, and 
finally reach the destination host.
The following figure shows the sub-net classification of a Class B IP address, in 
which a sub-net mask consists of a string of continuous “1s” and a string of 
continuous “0s”. The 1s corresponds to the network ID field and the sub-net 
number field, while the 0s correspond to the host ID field.
Figure 118   Sub-net classification of IP address
Classification of one more sub-net number field is at a price. For example, an IP 
address of class B originally consists of 65534 host IDs. But after a 6-bit-long 
sub-net field is classified, there may be at most 62 sub-nets (excluding sub-nets 
whose numbers are purely 1s or purely 0s). Each sub-net has 10bit host ID, i.e. 
each sub-net has 1022 host IDs at most. Totally, there are 62*1022=63364 host 
IDs which is less than the sum before sub-net classification.
If there is no sub-net division in an enterprise, then its sub-net mask is the default 
value and the length of “1” indicates the net-id length. Therefore, for IP addresses 
of classes A, B and C, the default values of corresponding sub-net mask are 
255.0.0.0, 255.255.0.0 and 255.255.255.0 respectively.
A router connecting multiple sub-nets will have multiple sub-net IP addresses. The 
IP addresses mentioned above cannot be directly used in communication, because:
■An IP address is only an address of a host in the network layer. To send the data 
messages transmitted through the network layer to the destination host, 
physical address of the host is required. So the IP address must be first resolved 
into a physical address.
■IP address is hard to remember, but a host domain name will be much easier to 
remember and is also more popular. So the host domain name must also be 
resolved into an IP address.
The following figure illustrates relation between host name, IP address and 
physical address.
net-id
host-id
net-id host-id
     11111111           11111111          111111 00     00000000
Local distribution
Subnet-id
Subnet IDHost ID (a)
(c) Class B address
Add subnet
number field
Subnet mask(b) 
						

							320CHAPTER 20: CONFIGURING IP ADDRESS
Figure 119   Relation between host name, IP address and physical address
Configure IP AddressConfigure IP Address for an Interface
Use a mask to label the network ID contained in an IP address. Example: the IP 
address of an Ethernet interface of a router is 129.9.30.42, and the mask is 
255.255.0.0. To obtain the network ID a logical and operation is performed 
between the IP address and the mask. Thus the above router’s Ethernet interface 
will be 129.0.0.0
Each interface of a router can have several IP addresses, among which one is the 
master IP address and the others are slave IP addresses. Any two IP addresses of a 
router cannot be in the same network segment.
Perform the following configuration in interface view.
1Configure master IP address of an interface
For each interface of a router, multiple IP addresses can be configured, among 
which one is the master IP address and the rest are slave IP addresses. Two IP 
addresses of one router can never be configured within the same network 
segment.
Ta b l e 352   Configure master IP address of an interface
By default, the interface has no master IP address.
OperationCommand
Configure master IP address of an interfaceip address ip-address { mask | 
mask-length }
Delete IP address of an interfaceundo ip address ip-address { mask | 
mask-length } 
						

							IP Address Overview321
When configuring the master IP address for an interface, note:
■An interface can only have one master IP address. 
■When deleting the IP address of the interface, if no IP address and mask is 
specified, all the IP addresses (including all slave IP addresses) will be deleted 
from the interface.
■One router can be configured with up to 200 IP addresses at most. 
■Only the Loopback interface can be configured with 32-bit mask and other 
interfaces can only be configured with 30-bit mask at most. 
2Configure slave IP address of an interface
Besides the master IP address, at most 4 slave IP addresses can be configured on 
an interface. The purpose of assigning slave IP addresses is to have the same 
interface located in different sub-nets, so as to create network routes with the 
same interface as the output port, and set up connection via the same interface to 
multiple sub-nets.
Ta b l e 353   Configure slave IP address of an interface
By default, the interface has no slave IP address. 
When configuring slave IP addresses for an interface, please note:
■Slave IP addresses cannot be on the same network segment with each other 
and they cannot be on the same network segment with the master IP address. 
Otherwise, the system will prompt:
■IP address configured now conflicts with others.
■If the interface is not configured with the master IP address, the first configured 
IP address will become the master IP address automatically.
■When there are slave IP addresses on the interface, the master IP address 
cannot be deleted. Otherwise, the system will prompt:
Must delete secondary before deleting primary.
3Set negotiable attribute of an IP address for an interface
When an interface is encapsulated with PPP, but not configured with IP address 
while the peer has been configured with IP address, the user can configure 
negotiable attribute of IP address on the interface on the local router. (To configure 
ip address ppp-negotiate command on the local router, and to configure 
remote address on the peer router) In this case, the local router can accept the IP 
address originated from PPP negotiation and allocated by the peer router. Such 
configuration is mainly used to obtain IP address allocated by ISP when accessing 
the Internet via ISP.
OperationCommand
Configure slave IP address of an interfaceip address ip-address { mask | 
mask-length } sub
Delete slave IP address of an interfaceundo ip address ip-address { mask 
| mask-length } [ sub ] 
						

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Ta b l e 354   Set negotiable attribute of IP address for an interface
By default, the interface has no negotiating IP address.
Note the following:
■Because PPP supports IP address negotiation, IP address negotiation of an 
interface can be set only when the interface is encapsulated with PPP. When 
the PPP is DOWN, the IP address originated from negotiation will be deleted. 
■If the interface has an original address, then after setting IP address of the 
interface to negotiable, the original IP address will be deleted.
■After setting IP address of an interface to negotiable, it is unnecessary to 
configure IP address for the interface, as negotiation will automatically 
originate an IP address.
■After setting IP address of an interface to negotiable, if the interface is set to 
negotiable again, then the IP address originated from the original negotiation 
will be deleted, and the interface obtains IP address through the 
re-negotiation.
■The interface will have no address after the negotiation address is deleted.
Configure IP Address 
Unnumbered for an 
InterfaceIntroduction to IP address unnumbered
Borrowing IP address will save IP address resources. If an interface has no IP 
address, it can neither generate any route nor forward any message. “IP Address 
Unnumbered” is used when you want to use an interface with no IP address. In 
such case, an IP address will be borrowed from another interface. If the lending 
interface has multiple IP addresses, then only the master one can be borrowed. 
However, if the lending interface has no IP address, then the IP address of the 
borrowing interface is 0.0.0.0. This function is implemented through the 
command ip address unnumbered.
Note the following:
■The borrower can not be an Ethernet interface
■The address of the lending interface cannot be lent by the borrowed interface.
■The lending interface can lend its address to multiple interfaces.
Because the borrowing interface has no IP address of its own, and can not route, 
two routes need to be configured manually to connect routers. 
IP address unnumbered configuration includes: 
■Activate/deactivate IP address unnumbered.
1Activate/deactivate IP address unnumbered
Perform the following task in the interface view,
OperationCommand
Set PPP negotiable attribute of IP address for an 
interfaceip address ppp-negotiate
Cancel negotiable attribute of IP address for an 
interfaceundo ip address 
ppp-negotiate
Assign IP address for the peer interfaceremote address { ip-address 
| pool [ pool-number ] }
Cancel IP address for the peer interfaceundo remote address 
						

							IP Address Overview323
Ta b l e 355   Configure IP address unnumbered
By default, the interface has no IP address.
2 Display IP address unnumbered
Ta b l e 356   Display IP address unnumbered
Configuration ExampleI. Configuration Requirements
Suppose the headquarters of a company is in Beijing, with subsidiary offices in 
Shenzhen and Shanghai and one office in Wuhan. R is the headquarters router, 
which connects the subsidiaries and office routers R1, R2 and R3 via PSTN. The 
four routers R, R1, R2 and R3 all have its serial port for dialing and one Ethernet 
interface to connect with local network.
II. Networking Diagram
Figure 120   Networking diagram of IP address unnumbered configuration 
III. Configuration Procedure
1Configure headquarters router R
[Router-Ethernet0] ip address 172.16.10.1 255.255.255.0
aBorrow IP address of Ethernet interface 0:
[Router-Serial0] ip address unnumbered Ethernet0 
[Router-Serial0] link-protocol ppp
bConfigure routing to Ethernet segment of Shenzhen router R1:
[Router] ip route-static 172.16.20.0 255.255.255.0 172.16.20.1
cConfigure the interface routing to Shenzhen router R1 serial port
[Router] ip route-static 172.16.20.1 255.255.255.255 serial0
OperationCommand
Activate IP address unnumberedip address unnumbered 
interface-type 
interface-number
Deactivate IP address unnumberedundo ip address unnumbered
OperationCommand
Display information of interface borrowing IP 
addressdisplay interfaces [type 
number]
Display the current configuration information.display current-configuration
Quidway
Quidway router Quidway
Quidway R1 ( Shenzhen)
R2 (Shanghai)
R3 ( Wuhan )R(Beijing
Headquarter)
PSTN
Router
Router
Router
Router 
						

							324CHAPTER 20: CONFIGURING IP ADDRESS
2Configure router R1 of Shenzhen subsidiary:
[Router-Ethernet0] ip address 172.16.20.1 255.255.255.0
aBorrow IP address of Ethernet
[Router-Serial0] ip address unnumbered Ethernet0 
[Router-Serial0] link-protocol ppp
bConfigure routing to Ethernet segment on Beijing router R, this routing is 
default routing
[Router] ip route-static 0.0.0.0 0.0.0.0 172.16.10.1
cConfigure interface routing to serial port of Beijing router R
[Router] ip route-static 172.16.10.1 255.255.255.255 serial0
Two static routing must be configured on Beijing headquarters router R to ensure 
access to Ethernet host of Shenzhen router R1.
The first static routing is to Ethernet segment of R1: the next hop is the IP address 
of serial port of R1 (or an unnumbered IP address)
ip route-static 172.16.20.1 255.255.255.0 172.16.20.1
The second static route is an interface route to the serial port of R1, and the next 
hop is the serial port of R
ip route-static 172.16.20.1 255.255.255.255 serial 0
After the two routes are added, router R will be able to forward the IP message to 
R1 correctly
Similarly, two static routes must be configured on R1 to access the Ethernet 
segment of router R. The first static routing is to Ethernet segment of R: the next 
hop is the IP address of serial port of R (or an unnumbered IP address)
ip route-static 0.0.0.0 0.0.0.0 172.16.10.1
The second static route is an interface route to the serial port of R, and the next 
hop is the serial port of R1.
ip route-static 172.16.10.1 255.255.255.255 serial0
The configuration of R2 and R3 is similar to that of R1.
Troubleshooting IP 
Address ConfigurationA router is a network interconnection device. So when IP address for an interface 
is configured, networking requirements and sub-net classification should be 
known. Normally, the following rules should be observed:
■The master IP address of a router Ethernet interface must be in the same 
network segment with the LAN to which this Ethernet interface is connected.
■Serial port IP addresses of the routers at both ends of WAN must be in the 
same network segment.
Fault 1: The router cannot ping through a certain host in LAN
Troubleshooting; First check if the IP address configuration of the routers Ethernet 
interface and the host in LAN are in the same network segment
If the configuration is correct, turn on the arp debugging switch on the router, and 
check if the router can correctly send and receive arp messages. If it can send but  
						

							Map between WAN Interface IP Address and Link Layer Protocol Address 325
cannot receive the arp message, then possibly the error is on the Ethernet physical 
layer.
Fault 2: When the interface is encapsulated with PPP or Frame Relay, the 
link layer protocol status does not change to UP.
Troubleshooting: check whether the IP address of this interface is in the same 
network segment as the opposite side.
Fault 3: After the interface borrows an IP address, the link layer protocol 
status will turn to UP, but it can not ping through itself, and other ports 
can not ping through this borrowed IP address either.
Troubleshooting: Check whether the lender port is UP. Only when the port 
protocol of the lender is UP, will the address be added to the route table and 
pinged through by other ports. 
Map between WAN 
Interface IP Address 
and Link Layer 
Protocol Address In a router, you shall maintain both the mapping from an Ethernet interface IP 
address to an MAC address, and that from a WAN interface IP address to a link 
layer protocol address. Namely there are the following types: 
■On a dialup interface (such as an asynchronous serial port or ISDN interface), 
mapping between IP address and dialing serial port is maintained by the 
command dialer route ip.
■On an interface encapsulated with X.25, the mapping between an IP address 
and X.121 address is maintained by the command x25 map ip.
■On an interface encapsulated with Frame Relay, mapping between an IP 
address and a virtual circuit number (DLCI) is maintained by the command fr 
map ip.
The above mapping tables are also called second routing tables, which are 
essential for the normal working of the router. For details, refer to related chapters 
in Link Layer Protocol. 
						

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