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Valid & Invalid IP Subnetting Question?

ericleggesmith
Beginner
Beginner

On reading various 'expert' networking text books, I seem unable to obtain a clear explanation of what occurs when 'invalid' or 'not allowed' IP addresses are used configured on interfaces and operational in a routed environment.

According to the text books, using a class B network 172.16.0.0 with a 2

bit SNM (2bits in 3rd Octet) , gives two valid nets, 172.16.64.0 and 172.16.128.0 each with 16382 hosts. i.e.

Network1 172.16.64.0

First Host 172.16.64.1

Last Host 172.16.127.254

Broadcast 172.16.127.255

Network2 172.16.128.0

First Host 172.16.128.1

Last Host 172.16.191.254

Broadcast 172.16.191.255

By implication, the addresses 172.16.0.0 through 172.16.63.255 and

172.16.192.0 through 172.16.255.255 are invalid as the SNM bit's are either

all off (0.0-63.255) or all on (192.0-255.255).

To test the theory that the invalid network addresses would not work in a

routed environment, I built the following test net:

LAN A - 172.16.0.0 with two hosts 172.16.0.2 and 172.16.0.3 and router A Ethernet port 172.16.0.1 all with SNM 255.255.192.0

WAN - 172.16.10.0 with router A Serial port 172.16.10.1 and router B serial port 172.16.10.2 both with SNM 255.255.192.0

LAN B - 172.16.200.0 with two hosts 172.16.200.2 and 172.16.200.3 and router B Ethernet port 172.16.200.1 all with SNM 255.255.192.0

According to the text books, all three networks are technically invalid, given the SNM, as all the addresses exist below 172.16.64.0 or above 172.16.192.0 and therefore all SNM bits are either all off or all on!

However, both Cisco's allowed the configuration to be entered, RIP

redistributed the 172.16.0.0 network and published the route as being

variably subnetted with one mask and three subnets (0.0, 10.0 and 200.0)

and all hosts pinged correctly.

My question is ...are the 'invalid' networks truely invalid?

If so ...

Why does the IOS allow the configuration to be entered ?

Why are the 'invalid' routes redistributed and routing appear to work?

Is the recommendation not to use these 'invalid' network numbers just that i.e. a recommendation, or can the 'invalid' nets actually be successfully deployed in a routed internetwork?

5 Replies 5

mark-obrien
Enthusiast
Enthusiast

Many texts disallow subnets of all zeroes and all ones for the reason that an all zeroes subnet includes the network address of the full, classful network, and the all ones subnet would include the broadcast address of the full network. In your example, the ".0" subnet has the two subnet bits set to zero, the ".64" subnet has "01" in the subnet portion, the ".128" has "10" in the subnet portion, and ".192" has "11 in the subnet.

The Cisco IOS command "ip subnet-zero" allows the use of subnets having all zeroes and all ones. This recognizes that, in most instances (all instances that I've ever seen, BTW), the classful network's network address and broadcast address are not used by any process. If you check the full configurations of your routers, I'm sure you'll see this command in both of them.

HTH,

Mark

wkumari
Beginner
Beginner

The restriction of not using the networks with all zeros and all ones was specified in RFC950 (Internet Standard Subnetting Procedure) :

This means the values of all zeros and all ones in the subnet field

should not be assigned to actual (physical) subnets.

This has been superceded by RFC1878 () which states (rather grumpily I have always felt!):

For the sake of completeness within this memo, tables 2-1 and 2-2

illistrate some options for subnet/host partions within selected

block sizes using calculations which exclude all-zeros and all-ones

subnets [2]. Many vendors only support subnetting based upon this

premise. This practice is obsolete! Modern software will be able to

utilize all definable networks.

Cisco IOS (>12.0 I believe) follows RFC1878 by default. To get back to the older (RFC950) scheme, try "no ip subnet-zero"

--Warren.