IP Address Overview
319
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 ID
Host ID
(a)
(c)
Class B address
Add subnet
number field
Subnet mask
(b)
Summary of Contents for Router 3032
Page 1: ...http www 3com com 3Com Router Configuration Guide Published March 2004 Part No 10014299...
Page 4: ...VPN 615 RELIABILITY 665 QOS 681 DIAL UP 721...
Page 6: ...2 ABOUT THIS GUIDE...
Page 7: ...I GETTING STARTED Chapter 1 3Com Router Introduction Chapter 2 3Com Router User Interface...
Page 8: ...4...
Page 16: ...12 CHAPTER 1 3COM ROUTER INTRODUCTION...
Page 34: ...30...
Page 60: ...56 CHAPTER 3 SYSTEM MANAGEMENT...
Page 98: ...94 CHAPTER 6 DISPLAY AND DEBUGGING TOOLS...
Page 110: ...106...
Page 114: ...110 CHAPTER 8 INTERFACE CONFIGURATION OVERVIEW...
Page 158: ...154 CHAPTER 10 CONFIGURING WAN INTERFACE...
Page 168: ...164...
Page 188: ...184 CHAPTER 13 CONFIGURING PPPOE CLIENT...
Page 192: ...188 CHAPTER 14 CONFIGURING SLIP Router ip route static 0 0 0 0 0 0 0 0 10 110 0 1...
Page 248: ...244 CHAPTER 16 CONFIGURING LAPB AND X 25...
Page 320: ...316...
Page 330: ...326 CHAPTER 20 CONFIGURING IP ADDRESS...
Page 362: ...358 CHAPTER 21 CONFIGURING IP APPLICATION...
Page 374: ...370 CHAPTER 23 CONFIGURING IP COUNT...
Page 406: ...402 CHAPTER 25 CONFIGURING DLSW...
Page 408: ...404...
Page 452: ...448 CHAPTER 29 CONFIGURING OSPF...
Page 482: ...478 CHAPTER 30 CONFIGURING BGP...
Page 494: ...490 CHAPTER 31 CONFIGURING IP ROUTING POLICY...
Page 502: ...498...
Page 508: ...504 CHAPTER 33 IP MULTICAST...
Page 514: ...510 CHAPTER 34 CONFIGURING IGMP...
Page 526: ...522 CHAPTER 36 CONFIGURING PIM SM...
Page 528: ...524...
Page 532: ...528 CHAPTER 37 CONFIGURING TERMINAL ACCESS SECURITY...
Page 550: ...546 CHAPTER 38 CONFIGURING AAA AND RADIUS PROTOCOL...
Page 590: ...586 CHAPTER 40 CONFIGURING IPSEC...
Page 599: ...IX VPN Chapter 42 Configuring VPN Chapter 43 Configuring L2TP Chapter 44 Configuring GRE...
Page 600: ...596...
Page 638: ...634 CHAPTER 43 CONFIGURING L2TP...
Page 649: ...X RELIABILITY Chapter 45 Configuring a Standby Center Chapter 46 Configuring VRRP...
Page 650: ...646...
Page 666: ...662...
Page 670: ...666 CHAPTER 47 QOS OVERVIEW...
Page 700: ...696 CHAPTER 49 CONGESTION MANAGEMENT...
Page 706: ...702 CHAPTER 50 CONGESTION AVOIDANCE...
Page 707: ...XII DIAL UP Chapter 51 Configuring DCC Chapter 52 Configuring Modem...
Page 708: ...704...
Page 762: ...758 CHAPTER 52 CONFIGURING MODEM...