Installation Guide of

Wireless Internet Access Server

WRT-2110


Appendix C. IP Address and Net-mask

C.1 IP address and netmask

The IP address netmask is used to divide an IP address into a network portion and a station portion. Certain symptoms may be unexpectedly observed if the IP netmasks of PC LAN stations and the WRT-2110 router are not set to the same value. This happens because PCs and the router are assigned to different IP sub-nets.

For example:

PC1, IP address 203.70.248.56, netmask 255.255.255.192 (length of 26),

PC2, IP address 203.70.248.88, netmask 255.255.255.192 (length of 26),

WRT-2110, IP address 203.70.248.93, netmask 255.255.255.0 (length of 24)

In this example, PC2 is in the same sub-net as WRT-2110, but PC1 is belonging to another sub-net. Both PC1 and PC2 can communicate with WRT-2110, but PC1 is unable to communication with PC2. WRT-2110 considers both PC1 and PC2 in the same sub-net, and therefore will not forward traffic between PC1 and PC2.

Take another example:

PC1, IP address 203.70.248.56, netmask 255.255.255.0 (length of 24),

PC2, IP address 203.70.248.88, netmask 255.255.255.0 (length of 24),

WRT-2110, IP address 203.70.248.93, netmask 255.255.255.192 (length of 26)

In this example, both PC1 and PC2 think WRT-2110 is in their sub-net. But WRT-2110 thinks PC2 is in its sub-net, but PC1 is not. There is no communication problem between PC2 and WRT-2110. On the other hand, if PC1 sends an IP packet (eg. SNMP request) to WRT-2110, WRT-2110 will respond, but the response packet is sent toward the WAN port unless a static route is manually added at WRT-2110 to point the PC1's sub-net to the Ethernet. For PC1 to access Internet through WRT-2110, similar measures should be taken to correct the IP address assignment problem.

Another noticeable side effect is that any traffic to PC1 from PCs on the LAN will cause WRT-2110 to send the packets to the WAN port all the time. Again, unless similar measures are taken to correct the problem, the WAN port will always send packets without any sense.


 

C.2 Explaining the Mask IP fields

The subnet mask fields are usually of the form, 255,255,255,XXX for private network used. The subnet mask fields in the setup screen show a digital representation of this IP address. The mask fields are broken up into 4 sections

To simplify this selection a number is displayed to the right/left of this box that tells you how many boxes/bits are selected. For quick reference the table below shows the most common subnets used and their corresponding selection number.

Subnet Mask

IP Class Number of IP in this Mask

255.0.0.0

A

254^3

255.255.0.0

B

254^2

255.255.255.0

C

254

255.255.255.128

1/2 C

126

255.255.255.192

1/4 C

62

255.255.255.224

1/8 C

30

255.255.255.240

1/16 C

14

255.255.255.248

1/32 C

6

255.255.255.252

-

2

Figure 1. Use this table to find an IP mask's corresponding IP number.

To sum up, the Internet Access Server can be used to separate the subnet mask through the parameters above after the NAT (Network Address Translation) is turned off.

Please refer to CH 6 Routing for more or consult your Internet Service Provider about your net mask setup..

 

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