VLSM

oconn11 · December 2004

Does anyone have a good way to explain variable length sub-masking? I have serached the forum and the net. I can understand what is going on from the examples I have read but the 'light bulb' is not going off in my head. I cannot 'see' how to do it on my own. Thank you.

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Ricka182 · December 2004

Variable Length Subnet Masks In Use

netbits   subnet mask	    total addresses      last octet values

/20	  255.255.240.0	         4096
/21	  255.255.248.0	         2048
/22	  255.255.252.0	         1024
/23	  255.255.254.0	          512
/24	  255.255.255.0	          256
/25	  255.255.255.128         128            0, 128

/26	  255.255.255.192          64            0, 64, 128, 192

/27	  255.255.255.224          32            0, 32, 64, 96, 128,
                                                 160, 192, 224

/28	  255.255.255.240          16            0, 16, 32, 48, 64, 80,
                                                 96, 112, 128, 144, 160,
                                                 176, 192, 208, 224, 240

/29    255.255.255.248           8            0,8,16,24,32,40,48,56,64,
                                                 72,80,88,96,104,112,120,
                                                 128,136,144,152,160,168,
                                                 176,184,192,200,208,216,
                                                 224,238,240,248

The "last octet value" column defines the valid starting point for networks that have less than 256 addresses. For instance, if you are in network 128.174.5.x and have a network netbit of /28, then your subnet mask is 255.255.255.240. In this example x could take on the values of 0 or 16 or 32 or 48 or 64, etc. If you have been assigned the block of addresses starting at 128.174.5.48, then your range would extend to 128.174.5.63. This would give you a total of 16 IP addresses for your network. However, under this scenario, address 128.174.5.48 would be your network address, 128.174.5.63 would be your broadcast address, and typically 128.174.5.49 would be the address of your router. So in fact you have the ability to place 13 devices on your network (16-3). 
In another example, if your network is 128.174.5.0 and your subnet mask is 255.255.255.0 then you would think that you have the ability to place 256 devices on your network. However, the lowest address in your range defines your network, the highest is the broadcast address, and a third address (usually the second lowest) is reserved for the router. This means you have a practical limit of 253 devices (256-3=253) on your network.

Maybe this can help a bit.....

wildfire · December 2004

Try this explantion here, refers to it in a simple way

http://www.kccommunications.com/htdocs/IP%20addressing%20summary%20notes.htm

oconn11 · December 2004

Thank you will give it a look.

Skywalker_ · December 2004

Ricka182 wrote:

....
For instance, if you are in network 128.174.5.x and have a network netbit of /28, then your subnet mask is 255.255.255.240. In this example x could take on the values of 0 or 16 or 32 or 48 or 64, etc. If you have been assigned the block of addresses starting at 128.174.5.48, then your range would extend to 128.174.5.63. This would give you a total of 16 IP addresses for your network. However, under this scenario, address 128.174.5.48 would be your network address, 128.174.5.63 would be your broadcast address, and typically 128.174.5.49 would be the address of your router. So in fact you have the ability to place 13 devices on your network (16-3)..

128.174.5.0 - 128.174.5.15 and 128.174.5.240 - 128.174.5.255 subnets are not counted as 128.174.5.0 and 128.174.5.255 are network and broadcast addresses for 128.174.5 network respectively. So, the total number of subnets is 2^No_of_bits_borrowed - 2 and in this case it's 2^4 - 2 = 14

There could also be a tricky question asking to recognize valid host addresses from the list and give ones which are in the first and the last subnets.

oconn11 · December 2004

Skywalker, you describe normal subnetting perfedtly. But how about sub-subnetting. Where would you go next with your example?? Thank you

Skywalker_ · December 2004

Skywalker_ wrote:

Ricka182 wrote:

....
For instance, if you are in network 128.174.5.x and have a network netbit of /28, then your subnet mask is 255.255.255.240. In this example x could take on the values of 0 or 16 or 32 or 48 or 64, etc. If you have been assigned the block of addresses starting at 128.174.5.48, then your range would extend to 128.174.5.63. This would give you a total of 16 IP addresses for your network. However, under this scenario, address 128.174.5.48 would be your network address, 128.174.5.63 would be your broadcast address, and typically 128.174.5.49 would be the address of your router. So in fact you have the ability to place 13 devices on your network (16-3)..

128.174.5.0 - 128.174.5.15 and 128.174.5.240 - 128.174.5.255 subnets are not counted as 128.174.5.0 and 128.174.5.255 are network and broadcast addresses for 128.174.5 network respectively. So, the total number of subnets is 2^No_of_bits_borrowed - 2 and in this case it's 2^4 - 2 = 14

There could also be a tricky question asking to recognize valid host addresses from the list and give ones which are in the first and the last subnets.

Correction: According to Cisco all -zeros and all-ones subnets are allowed.

RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete! Modern software will be able to utilize all definable networks."

and from Cisco's website:

Subnet-Zero

Using subnet zero for addressing was discouraged because of the confusion inherent in having a network and a subnet with indistinguishable addresses.

With reference to our example above, consider the IP address 172.16.1.10. If you calculate the subnet address corresponding to this IP address, the answer you arrive at is subnet 172.16.0.0 (subnet zero). Note that this subnet address is identical to network address 172.16.0.0, which was subnetted in the first place, so whenever you perform subnetting, you get a network and a subnet (subnet zero) with indistinguishable addresses. This was formerly a source of great confusion.

Prior to Cisco IOS® Software Release 12.0, Cisco routers, by default, did not allow an IP address belonging to subnet zero to be configured on an interface. However, if a network engineer working with a Cisco IOS software release older than 12.0 finds it safe to use subnet zero, the ip subnet-zero command in the global configuration mode can be used to overcome this restriction. As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default, but if the network engineer feels that it is unsafe to use subnet zero, the no ip subnet-zero command can be used to restrict the use of subnet zero addresses.

In versions prior to Cisco IOS Software Release 8.3, the service subnet-zero command was used.
The All-Ones Subnet

Use of the all-ones subnet for addressing has been discouraged in the past because of the confusion inherent in having a network and a subnet with identical broadcast addresses.

Skywalker_ · December 2004

oconn11 wrote:

Skywalker, you describe normal subnetting perfedtly. But how about sub-subnetting. Where would you go next with your example?? Thank you

Do you have to know sub-subnetting for CCNA exam? I don't think you do.
For real world problems you would be using network/subnet calculators instead pencil and paper.

oconn11 · December 2004

I am in CISCO Networking Academy third semester. We are starting RIP2 and OSPF and both involve sub-subnetting. Thank you

Skywalker_ · December 2004

oconn11 wrote:

I am in CISCO Networking Academy third semester. We are starting RIP2 and OSPF and both involve sub-subnetting. Thank you

At home, I have 10.1.5.0/24 private network. Its default netmask is 8 bits because it's class A network.
On my network, I am using /24 subnet mask and only last 8 bits are used for the hosts on each sub2-subnet. Everything in between eg 2nd and 3rd octets are subnets and sub-subnets and then I further subnetted ( subnetted twice sub2-subnet or whatever the correct term is used) these sub-subnets using /24 subnet mask.

I think that's what it is. Someone else on this forum could clarify this.

If you are attending cisco classes could you ask your lecturers/tutors how should we count the number of subnets? should all-zero and all-ones (as in my original example) be counted or not? For the purpose of CCNA exam.

THanks

VLSM

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