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...

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...

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...

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...

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...

322CHAPTER 20: CONFIGURING IP ADDRESS
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...

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...

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...

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,...

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