vlsm

[fishintaiwan]

192.168.5.X / 23

is this MASK can be work?
my router don't warning me any error.

Cisco IOS Software, 2800 Software (C2800NM-ADVENTERPRISEK9-M), Version 12.4(17),
RELEASE SOFTWARE (fc1)
Cisco 2811 (revision 53.51) with 249856K/12288K bytes of memory.
Processor board ID FTX1143A47L

Router#sh protocols fastEthernet 0/1
FastEthernet0/1 is administratively down, line protocol is down
Internet address is 192.168.5.5/23

[lammle]

the rfc's are pretty clear:

a /23 can be used to address hosts for class A and Class B. Class C hosts start at /24.

yes, for some reason the router can take a mask like that and cover more hosts, but it is not recommended. That is a poor design. I am not sure why cisco allows this, but I have seen it before. Also, I was contracted to a company that had a mask similar to this design and they had a lot of problems. I redesigned and implemented a correct network and it fixed all their problems, so although you can do it, I wouldn't do it.

A /23 can be used with class C when you are summarizing, that is acceptable, but again, not for host addressing. And of course, the CCNA objectives would never allow a class C /23 mask...

Cheers!
Todd Lammle

[HermeszData]

the rfc's are pretty clear:

a /23 can be used to address hosts for class A and Class B. Class C hosts start at /24.

yes, for some reason the router can take a mask like that and cover more hosts, but it is not recommended. That is a poor design. I am not sure why cisco allows this, but I have seen it before. Also, I was contracted to a company that had a mask similar to this design and they had a lot of problems. I redesigned and implemented a correct network and it fixed all their problems, so although you can do it, I wouldn't do it.

A /23 can be used with class C when you are summarizing, that is acceptable, but again, not for host addressing. And of course, the CCNA objectives would never allow a class C /23 mask...

Cheers!
Todd Lammle

Todd,
I think I understand subnetting fairly well, but the Class C summarization examples have me a bit confused.

Why are the maskes different? In the examples on p.148/149 you have a network addresses of:
192.168.16.0 255.255.255.240 (.0/2
192.168.144.0 /20 (summarization address/mask

In both cases the summarization mask you list is the same - 255.255.240.0

WHY would the mask NOT be 255.255.255.240 ?????

How does the Summarization Mask differ from the Network Subnet Mask and WHY????

One Completely Confused Hermesz Data!!

Thanks in advance,

John

EDIT: This question is only concerning Class C implementation!

[Fuzz]

Summarisation is combining networks together for simpler routing updates, it's also called supernetting, the opposite of subnetting. When you subnet a class C address, you make multiple subnets with smaller host ranges by using larger subnet masks. With summarisation you do the exact opposite. By using smaller subnet masks you are collating the subnets under one address. All hosts that belong to any subnet within that supernet are considered on the same network, making routing updates smaller by sending onlt one route for multiple addresses.

If the mask used to summarise was the same as the subnet mask, you wouldn't be summarising anything, only referencing the subnet itself.