Looking at all possible subnet masks, you get:
subnet mask | # subnets | # hosts/subnet | total # hosts | usage -------------+------------+----------------+---------------+------ 1:7 | not allowed| | | 2:6 | 2 | 62 | 124 | 49 % 3:5 | 6 | 30 | 180 | 71 % 4:4 | 14 | 14 | 196 | 77 % 5:3 | 30 | 6 | 180 | 71 % 6:2 | 62 | 2 | 124 | 49 % 7:1 | not allowed| | |
Just one probably silly question - why should the subnet mask 1:7 not be allowed? If I get it right, you speak of netmask like 255.255.255.128.
Your interpretation is correct. The mask 1:7 (as 7:1) is not allowed according to the recommendation to use all zeroes and all ones *neither* in the host part, *nor* in the subnet part.
From RFC1009 [Requirements for Internet gateways], page 6:
RFC1009> The bit positions containing this extended network number are RFC1009> indicated by a 32-bit mask called the "subnet mask" [21]; it is RFC1009> recommended but not required that the <Subnet-number> bits be RFC1009> contiguous and fall between the <Network-number> and the RFC1009> <Host-number> fields. No subnet should be assigned the value RFC1009> zero or -1 (all one bits).
Which should let you have two subnets of 126 hosts each.
Similar reasons might have lead to the following B) In order to prevent implementation problems, network numbers ending with 0 or 255 should NOT be reassigned. found in ripe-72.txt . Just walking off the topic ... Peter