- Here we introduce the GUI and command interface on Windows/Unix systems for retrieving/updating basic network parameters.
- Through the routing table, we discuss the how basic routing are done on a host machine.
- If you have a home PC (running XP) with Ethernet network interface, the important network parameters of your PC can be accessed by clicking on the start | control panel then choose network connections and then click on the local network connection.
- You will see the following status
window popup.
It shows the duration of the connection and the transmission speed (Fast Ethernet at 100Mbps). - Click on the properties button.
The following windows popup:
Click on the Internet (TCP/IP) and hit the properties button. - The following TCP/IP properties
window will show up.
- It shows the current PC is assigned with IP address 192.168.0.177.
- The subnet mask 255.255.255.0 indicates that the network address field of the IP address is 24 bits and host address field is 8 bits. The 3byte pattern in the network address field, 192.168.0, uniquely identified the subnet connected by this PC. The equivalent classless notation for the subnet is 192.168.0.0/24. /24 indicates the first 24 bits of the address pattern is for the network address field.
- The 192.168.x.x C class subnets is reserved for private LAN usage. The router in the Internet will not forward packet with 192.168.x.x as destination address.
- The default gateway is 192.168.0.1. Except for the packet for the local machine or for the subnet 192.168.0.0/24, all packets sent by this PC for other networks will be routed through the gateway machine with 192.168.0.1 IP address. This is typically a machine has dial up link and run Internet sharing. Other machine is the same subnet can request it to assign IP address and other netowrk setting such DNS servers using DHCP (Dynamic Host Control Protocl) protocol. The default gateway uses network address translation (NAT) to asign/translate the internal IP address and port used by the internal network to those used in the public Internet.
- We saw two DNS server entries there. The primary DNS server or Preferred DNS server is 128.198.1.250. This DNS server will be tried first for DNS doman name-IP address mapping request. If it fails, the PC will try the secondary DNS server or the Alternate DNS server.
- The following network interface and routing table information can be displayed by using "netstat -rn" on a cmd window (start | run then type "run")
- C:\Documents and Settings\Edward
Chow>netstat -rn
Route Table
===========================================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 50 56 c0 00 08 ...... VMware Virtual Ethernet Adapter (Network Address
Translation (NAT) for VMnet8)
0x3 ...00 50 56 c0 00 01 ...... VMware Virtual Ethernet Adapter (basic host-only
support for VMnet1)
0x4 ...08 00 46 63 b5 5d ...... Intel(R) PRO/100 VE Network Connection - Packet
Scheduler Miniport
===========================================================================
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.0.1 192.168.0.199 20
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.0.0 255.255.255.0 192.168.0.199 192.168.0.199 20
192.168.0.199 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.0.255 255.255.255.255 192.168.0.199 192.168.0.199 20
224.0.0.0 240.0.0.0 192.168.0.199 192.168.0.199 20
255.255.255.255 255.255.255.255 192.168.0.199 192.168.0.199 1
Default Gateway: 192.168.0.1
===========================================================================
Persistent Routes:
None - The first part of the routing table shows there are four netowrk intefaces. The first 0x1 is a TCP loop back interface. It is used to send back up the packets destined to the process on the same machine but a differnt port. For example, you run the browser and apache web server on the same machine for testing. The fourth 0x4 interface is an Intel Pro/100 Fast Ethernet card with 08 00 45 63 b5 5d as MAC address.
- The other two interfaces are used by VMWARE package for the virutal network emulation.
- The 2nd part of the routin gtable shows the routing table entries used the network software for routing decision.
- When you change the IP address, subnet mask, and default gateway address, the system will update the above routing table.
- When the IP software module receives the packet.
- It will take the destination IP address of the packet, perform the bit and operation with each of the netmasks.
- The resulting address values are then compared with the first columnn, network destination.
- If there is only one match, the packet will send out through the interface indicated by the matched routing entry.
- If there are more than one match, the one with longest prefix (longest network address field) in teh network destination column will be chosen.
- For example, the packet with 192.168.0.199 as destination address is received. After bit and operation on the netmasks, the resulting address values are 0.0.0.0, 192.0.0.0, 192.168.0.0, 192.168.0.199, 192.168.0.199, 192.0.0.0, 192.168.0.199. Among them, only the network destination addresses in the first routing entry, the 3rd, and the 4th entry will be matched exactly. But the third entry which has 192.168.0.199 has longest network address field (32 bits). Therefore the packet will route through the interface 127.0.0.1 (this unique address is called loop back IP address).
- If the packet with 128.198.1.250 as destination address is received. After bit and operation on the netmasks, the resulting address values are 0.0.0.0, 128.0.0.0, 128.198.1.0, 128.198.1.250, 128.198.1.250, 128.0.0.0, 128.198.1.250. Among them only the network address in the first routing entry matched with the corresponding masked addresses (0.0.0.0). Therefore the packet is sent through the network interface 192.168.0.199 with the destination MAC address of the default gateway 192.168.0.1. It will be relayed by the gateway to Internet.
- If the packet with 192.168.0.177 as destination address is received. After bit and operation on the netmasks, the resulting address values match only with the network destination values in the 1st and 3rd entry. Since the 3rd entry has longer network address field. The 3rd routing entry is selected. Here the gateway column has the same IP address value as the interface, since 192.168.0.177 is on the same subnet. ARP protocol will be used to find out the destination MAC address for sending out the Ethernet frame.
- 224.0.0.0 has the first 3 bit set. It is a multicast address.
- 5th entry has all host fields set to 1. It is a local broadcast address for subnet 192.168.0.0/24.
- 7th entry has all fields set to 1. It is a global broadcast address.
