Subnetting Woes

satishtech

Kindly correct my subnetting if its wrong:

Can any subnet mask be used with any IP irrespective
of Class, for example a Class C Subnet using a Class A
Mask.

193.10.15.140/9

/9 = 255.128.0.0

2^1 = 2 subnets
2^23 - 2 = 8388608 - 2 = 8388606 hosts per subnet

256-128 = 128

0 , 128

Subnet : 193.0.0.0 193.128.0.0
First IP: 193.0.0.1 193.128.0.1
Last IP: 193.127.255.254 193.255.255.254 (corrected )
Broadcast IP: 193.127.255.255 193.255.255.255(corrected)

thanks anurag my oversight

And , what is the smallest subnet and what is the largest subnet
/8 smallest ? /30 largest ?

CCIE Wanna Be

No, I'm pretty sure the base IP class subnet mask must be preserved. The above address is class C, so you can have /24 to /32 (though really only to /30). What you describe above would really be used in route summarization for routing protocols. This is beyond the scope of the Net+.

anuragaks10

satishtech wrote: »

Can any subnet mask be used with any IP irrespective
of Class, for example a Class C Subnet using a Class A
Mask.

Yes. This is what is referred to as CIDR (Classless Interdomain Routing) and it allows 'prefix-aggregation' / 'Summarization' / 'Supernetting' (same thing, different words).

No. of Hosts = 2^{no. of host bits} - 2 (don't forget the network and broadcast address!)

Last host IP address for the 2nd subnet should be : 193.255.255.254
Broadcast IP address for the 2nd subnet should be : 193.255.255.255

Keep in mind the octet you are working in during subnetting.

Smallest and Largest subnets ? Maybe I'm missing some context but theoretically it can be as small as /1 while largest can be /32 but that's not gonna be practical at all. Perhaps, someone else can shed more light on this.

satishtech

Dear CCIE ,(God I wish I have a CCIE)

I am also preparing for CCENT,I wonder if I will get subnet IP questions that dont preserve the base IP class subnet mask.

satishtech

For CCENT exam should I presume I will get questions that stick to the base IP class subnet mask ?For Network+ what should be my concern ?

/8 to /30 Class A
/16 to /30 Class B
/24 to /30 Class C

anuragaks10

I think he's just a CCIE-wannabe
You should really just work on subnetting without worrying about classes. Classless is to Classful addressing, what TCP/IP is to OSI. Hope that made sense :P
Learn basic subnetting for Net+ and in-depth subnetting for CCENT. Forget classes and their default mask while subnetting except what they have as their values by default (which really doesn't matter while subnetting). Meaning, what is class A, B, C and what are masks associated with A,B,C (both in public and private addressing).

Cheers

PS- What are you preparing for atm ? Network+ or CCENT ? or both ?

satishtech

Both Exams , but may take Network+ first.

193.10.15.140 /25
/25 = 255.255.255.128

How do I subnet this ?

anuragaks10

For 193.10.15.140/25
No. of subnets - 2¹ = 2
No. of hosts = 2⁷ - 2 = 126 (per subnet)

'Block size' = 128 hence the two subnets would be 193.10.15.0 and 193.10.15.128
193.10.15.0 :-
First host IP - 193.10.15.1
Last Host IP - 193.10.15.126
Broadcast IP - 193.10.15.127

193.10.15.128 :-
First Host IP - 193.10.15.129
Last Host IP - 193.10.15.254
Broadcast IP - 193.10.15.255

CCIE Wanna Be

satishtech wrote: »

For CCENT exam should I presume I will get questions that stick to the base IP class subnet mask ?For Network+ what should be my concern ?

/8 to /30 Class A
/16 to /30 Class B
/24 to /30 Class C

Yes and Yes. Also, CIDR notation is simply just a way to represent a subnet mask in short form e.g. (255.255.255.240 = /28.

Creating a subnet (which by definition is a subset of a classful network) with assignable host addresses is different than summarizing a list ip subnets to route traffic to or from. In example 193.0.0.0/8 in a routing table would match any address with the first octet as 193.x.x.x, however the assigned addresses on the interface would not have a /8 mask. When subnetting, the subnet can fall on class-full boundaries /16 or /24 or it cannot /9 /10, /17, /26. etc, but the base mask of the given ip network address must be preserved.

satishtech

another few examples (anurag)

129.11.5.200 /11
Class B IP with Class A Mask

10.0.0.0 /14
10.5.8.100/12
Class A IP's

Yes CCIE , base mask has to be preserved,looks like that.