User's Manual for
PLANET ISDN Router Product
IRT-401
Appendix A. 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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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:
128 64 32 16 8 4 2 1 added together = 2552nd example, the number 248 is represented by:
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:
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255 255 255 248
3rd example, the number 224 is represented by:
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:
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