- TCP/IP and IPX Routing Tutorial
- Appendix: Restrictions in the use of class C subnetting
The rules for forming an IP address include the following:
“IP addresses are not permitted to have the value 0 or -1 for any of the , , or fields (except in the special cases listed above [relating to broadcast or network addresses]). This implies that each of these fields will be at least two bits long.” [RFC 1716, Almquist & Kastenholz, p.45]
If this rule must be adhered to, the Netmask ask 255.255.255.128 cannot be used. This is used as the first example in the Tutorial to create two separate logical networks for the purpose of connecting a LAN to a WAN. This Netmask ask cannot be used because only one bit is reserved for the , and so it can only take on the value of 0 or -1 (being all one’s). However, it was found that most TCP/IP implementations do not seem to enforce this rule. This includes Microsoft TCP/IP for Windows 95 and NT, and SCO Unix. Novell NetWare Server’s TCP/IP however does insist that the not be -1, but it can be 0. This rule also implies that the use of the Netmask ask 255.255.255.192, which creates four distinct networks, only allows for the use of two. Writing this Netmask ask in hex, it is FFFFFFC0 which in binary is:
11111111 11111111 11111111 11000000. | |
At first, it appears that four subnet numbers are available: 00, 01, 10 and 11. However, since the rule says that it cannot be 0 or -1, only two subnet numbers are available: 01 and 10.
From the example in section 4 of the Tutorial, with a class C network number of 210.20.30, the following ranges are available for use:
Net# Address Range ---- ------------- ------------- 01 210.20.30.65 to 210.20.30.126 --> VALID 10 210.20.30.129 to 210.20.30.190 --> VALID
The following ranges are wasted:
Net# Address Range ---- ------------- ------------- 00 210.20.30.1 to 210.20.30.62 --> WASTED 11 210.20.30.193 to 210.20.30.254 --> WASTED
These do not include the network and broadcast addresses. The example on Page 8 can be made to work using the same Netmask ask of 255.255.255.192 if the TCP/IP implementation allows the use of the 0 subnet number. In this case, the only change is to use Net#2 as opposed to Net#3 for the WAN connection. Node C (WAN) can have IP address 210.20.30.130, and the gateway node can have IP address 210.20.30.190.
If the 0 subnet number cannot be used, the Netmask ask will have to change to FFFFFFE0, or 255.255.255.224, to give 3 subnet bits. The subnet numbers in binary are then: 000 001 010 011 100 101 110 111. The numbers 000 and 111 are illegal, leaving 6 networks. The valid IP addresses would then be:
Net# Address Range --- ------------- ------------- 001 210.20.30.33 to 210.20.30.62 010 210.20.30.65 to 210.20.30.94 011 210.20.30.97 to 210.20.30.126 100 210.20.30.129 to 210.20.30.158 101 210.20.30.161 to 210.20.30.190 110 210.20.30.193 to 210.20.30.222
These do not include the network and broadcast addresses. To implement this change in the example, map the IP addresses in each of the three networks to those in the table above. This will leave three networks unused. For example:
Node From To
------- ------------- -------------
A 210.20.30.1 210.20.30.33 (Net# 001)
B 210.20.30.2 210.20.30.34 (Net# 001)
C (AB) 210.20.30.10 210.20.30.40 (Net# 001)
C (DE) 210.20.30.70 210.20.30.65 (Net# 010)
C (WAN) 210.20.30.200 210.20.30.97 (Net# 011)
D 210.20.30.81 210.20.30.66 (Net# 010)
E 210.20.30.82 210.20.30.67 (Net# 010)
G 210.20.30.254 210.20.30.126 (Net# 011)
Depending on the operating systems or routers used, the Netmask ask 255.255.255.128 may or may not be acceptable. If at all possible, the 0 and -1 subnet numbers should be avoided. By following this rule, it should be possible to interchange router equipment within the network without having to change the addressing scheme in order to satisfy rules that may or may not be enforced.