| Author |
Is this subnetting right?
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| Spides 2003-02-25, 10:31 pm |
| MAybe it's late at night but I have a question and answer below. Private IP of 192.168.1.0/24 The answer states that subnet 255.255.255.192 allows 4 subnets with 64 clients each. I thought the subnet 192 is two 1's giving us 2 to the power makes 4 -2 giving 2 usuable subnets. With six zero's for the hosts giving 2 to the power of 6 is 64 -2 giving 62 usuable hosts per subnet. Perhaps I should go to bed. | |
| deadfred 2003-02-26, 5:44 am |
| quote:
Originally posted by Spides
I thought the subnet 192 is two 1's giving us 2 to the power makes 4 -2 giving 2 usuable subnets. With six zero's for the hosts giving 2 to the power of 6 is 64 -2 giving 62 usuable hosts per subnet. Perhaps I should go to bed.
According to my math ... you're right .... | |
| mawwoods 2003-02-26, 6:32 am |
| I await being shot down in flames on this one, but here goes:
subnet mask of 255.255.255.192 (equivalent of 192.168.1.0/26) gives last octet of:
1 1 0 0 0 0 0 0
The first two bits are network addresses, allowing four networks: 00,01,10,11
The last 6 bits can be used for host addresses, which allows 64 combinations per network. However, all 1 and all 0 host addresses are reserved, and cannot be used for hosts, which takes off two combinations, which means only 62 host addresses are available per network.
So I think you will be allowed 4 networks, and 62 hosts for each network with the subnet stated.
If anyone thinks this is wrong, please correct me, I'd like to know  | |
| deadfred 2003-02-26, 9:05 am |
| sorry .. I misread .... I saw a 4 in the origiinal post and then skimmed .... The subnets, allowable host address and broadcast addresses are as follows ..
192.168.1.0 192.168.1.1 192.168.1.62 192.168.1.63
192.168.1.64 192.168.1.65 192.168.1.126 192.168.1.127
192.168.1.128 192.168.1.129 192.168.1.190 192.168.1.191
192.168.1.192 192.168.1.193 192.168.1.254 192.168.1.255 | |
| mawwoods 2003-02-26, 10:30 am |
| I think I get what you mean, the ip addresses on the left are the network addresses, the middle two addresses are the range and the last address is the broadcast address? | |
| deadfred 2003-02-26, 12:46 pm |
| correct. | |
| Spides 2003-02-26, 3:10 pm |
| Good explanation Deadfred, but:
I thought in able to determine how many subnets you will have you, as in this example. See how many ones are in the field. in this case 192 = 128 + 64 so you have two, to the power of two is four - 2 for the network and broadcast addresses = 2 Subnets!If you had a 128 subnet that would make one subnet and 126 hosts.
Also the ranges you have done, doesn't the subnet range be added together until you reach the subnet number, in this case 192. So the last address you could have would be 190. So the range as you havve said would go 256-192=64 so the range 64 128 192 I thought it stopped at the subnet mask or can you go to 253 as the last available host. Good work Deadfred, just curious. I should get the books back out. | |
| luisjo 2003-02-26, 5:34 pm |
| Is that an 218 question????? | |
| mawwoods 2003-02-26, 5:46 pm |
| Spides, as I understand it rule of all 1's and 0's being reserved for broadcast and host addresses only applies to the host portion of the address.
Subnetting is the extension of the network portion of the address past the standard bit boundary, eg 26 bits as the network portion rather than the standard 24 for a class c.
I found it a bit easier when I started doing the calulations in binary. Not that I enjoy it, but it was designed for binary and I think it is easier to see how the rules work. | |
| luisjo 2003-02-26, 5:53 pm |
| Thanks for ignoring me | |
| Spides 2003-02-26, 6:23 pm |
| Luisjo, I don't think it's on the 70-218 exam, I did that exam but don't recall it. | |
| B4yaman3 2003-02-26, 10:37 pm |
| From what I know with a 192 subnet mask [class C]you can only get 2 subnets.The only how you can get 4;
bits mask subnets host maskbit
2 255.255.255.192 4 62 /26
But this is the /24
Class C Effec Effec
# bits Mask Subnets Hosts
------- --------------- --------- ---------
2 255.255.255.192 2 62
3 255.255.255.224 6 30
4 255.255.255.240 14 14
5 255.255.255.248 30 6
6 255.255.255.252 62 2
From Cisco's website..Effec= Effective | |
| mawwoods 2003-02-27, 4:27 am |
| Luisjo,
I wasn't ignoring you, I was writing my post when you posted yours...only saw yours the next day. | |
| mawwoods 2003-02-27, 4:44 am |
| Anyway, back to the topic in hand...this is all starting to confuse me so I have gone to have a look at the cisco website.
The page I found (using the keyword subnets), had a list of subnet tables. For a class C subnet of 255.255.255.192 (/26), it states 4 subnets are available.
I think the confusion may arise from the fact that while these are available, I think you have to enable the ip subnet-zero command to use them (default from IOS 12 onwards). | |
| edmonds_robert 2003-02-27, 9:30 am |
| The trick to this question is in the wording. Their example does indeed give you 4 subnets with 64 hosts, but they're not all useable by default. However, by using the ip subnet zero and ip classless commands, they ARE both useable. The firs and last address in the subnet rule still stands however. | |
| B4yaman3 2003-02-27, 1:30 pm |
| Yes edmonds_robert because the new IOS supports it.. | |
| Spides 2003-02-27, 5:20 pm |
| But this question was study for a Microsoft exam not a Cisco just to add more confusion. | |
| mawwoods 2003-02-27, 5:31 pm |
| Then I suppose the question goes from does Cisco support it, to what is theoretically feasable:
i.e. 4 networks, each with 62 hosts. | |
| edmonds_robert 2003-02-27, 5:39 pm |
| quote:
Originally posted by Spides
But this question was study for a Microsoft exam not a Cisco just to add more confusion.
That's OK because Windows 2000 support OSPF routing, which is a classless routing protocol and therefore should support the use of subnet zero and the all ones subnet. | |
| ccgolfer 2003-02-28, 4:31 am |
| absolutely correct edmonds_robert
Now try teaching that to a class in a 2 hour session like I have to | |
| djmaplethorpe 2003-02-28, 10:21 am |
| quote:
Originally posted by Spides
But this question was study for a Microsoft exam not a Cisco just to add more confusion.
I guess my question is why are we talking Microsoft on a CCNA page. It really gets confusing when you start mixing vendors and what you can and can't do. Even though Cisco allows for the subnet zero command to be used and through the magic of OSPF and classless routing more rules go out the window, you must understand that for purpose of the CCNA test the subnet zero doesn't come into play and the 192 mask will give you 2 subnets and 62 usable hosts. |
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