User's Manual  for
PLANET ISDN Router Product

IRT-401


Appendix A. IP Address and Netmask

  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 IRT-401 router are not set to the same value. This happens because PCs and IRT-401 are assigned to different IP sub-nets.

For example:

PC1, IP address 205.158.241.56, netmask 255.255.255.192 (length of 26),

PC2, IP address 205.158.241.88, netmask 255.255.255.192 (length of 26),

IRT-401, IP address 205.158.241.93, netmask 255.255.255.0 (length of 24)

In this example, PC2 is in the same sub-net as IRT-401, but PC1 is belonging to another sub-net. Both PC1 and PC2 can communicate with IRT-401, but PC1 is unable to communication with PC2. IRT-401 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 205.158.241.56, netmask 255.255.255.0 (length of 24),

PC2, IP address 205.158.241.88, netmask 255.255.255.0 (length of 24),

IRT-401, IP address 205.158.241.93, netmask 255.255.255.192 (length of 26)

In this example, both PC1 and PC2 think IRT-401 is in their sub-net. But IRT-401 thinks PC2 is in its sub-net, but PC1 is not. There is no communication problem between PC2 and IRT-401. On the other hand, if PC1 sends an IP packet (eg. SNMP request) to IRT-401, IRT-401 will respond, but the response packet is sent toward the WAN port unless a static route is manually added at IRT-401 to point the PC1's sub-net to the Ethernet. For PC1 to access Internet through IRT-401, 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 IRT-401 to dial or maintain the ISDN line up, because IRT-401 intends to forward the packet to WAN port. Again, unless similar measures are taken to correct the problem, the ISDN line may be staying up unexpectedly.

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wpe12.jpg (8434 bytes)

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  1. 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 NMS manager show a "binary" graphical representation of this IP address. The mask fields are broken up into 4 sections each containing 8 boxes. These 8 boxes represent the binary equivalent for each of the IP numbers.

For example, the number 255 is represented by:

wpe2.gif (1461 bytes)
128 64 32 16 8 4 2 1 added together = 255

2nd example, the number 248 is represented by:

wpe3.gif (1515 bytes)

128 64 32 16 8 4 2 1 added together = 248

Therefore, the IP Mask for an example of 255.255.255.248 will be:

¡@wpe6.gif (2299 bytes)   29

           255                     255                    255                      248

3rd example, the number 224 is represented by:

wpe7.gif (1481 bytes)

                  128 64 32 16 8 4 2 1 added together = 224

Therefore, the IP Mask for an example of 255.255.255.224 will be:

wpe9.gif (2366 bytes)   27

           255                     255                   255                       224

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

Equivalent number of bits

255.255.255.0

24

255.255.255.128

25

255.255.255.192

26

255.255.255.224

27

255.255.255.240

28

255.255.255.248

29

255.255.255.252

30

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




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Copyright (C) 1999 PLANET Technlogy Corporation