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TOPIC: is it possible 2 find Subnet Mask

is it possible 2 find Subnet Mask 5 years 7 months ago #36215

  • rizin
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Hi All,

I have one instant clarification that is it possible to find exact subnet mask of any give ip address ie.. say IP Address is 172.19.20.100

1) Subnet mask must be ascertain without knowing the network borrowed bits or subnet prefix length.

2) let me put in other words, just by seeing the IP Address can we able to find the exact subnet mask of the specific address.

Kindly Note: I would like to know the answer as the final solutions.

Thanks in advance,

Rizin
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Re: is it possible 2 find Subnet Mask 5 years 7 months ago #36216

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Nope. The only time there is something remotely like what your talking about is when you have a point to point link you can guess that it's going to be /30 because that would leave 2 usable hosts for each end(even then thats a guess that assumes best practices are in use). Any other time it's pretty much a crap shoot.


For example 172.19.20.100 Could be the subnet ID in the


/30 subnet where 172.19.20.101 and 172.19.20.102 are hosts and 172.19.20.103 is the broadcast address


or 172.19.20.100 could be the part of the host range on the

/18 subnet between 172.19.0.1 to 172.19.63.254 host range where 172.19.63.255 is the broadcast address and the subnet ID is 172.19.0.0
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Re: is it possible 2 find Subnet Mask 5 years 7 months ago #36217

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Thanks nevin,

So it is concluded that We need some sort of tip to ascertain the SUBNET MASK isnt'it ?

Nevin, i would like to ask one question other than our normal procedure, since you said POINT TO POINT, will we able to use different Network ID or Different Class address in the point to point link.

1) Like Router 1 belongs to Head office and Router 2 Belongs to Branch office.

2) Head office in US and Branch office in India.

3) At this Point ISP Providers are not the same.

4) So assume Head office ISP U.S gives One Static IP Addrress 10.20.40.95

5) Branch Office ISP India gives One static IP Address 195.155.34.26

6) Now at any cost we must connect the both R1&R2 routers POINT TO POINT.

7)Would you mind shed some light on this.

Thanks in Advance,

Rizin
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Re: is it possible 2 find Subnet Mask 5 years 7 months ago #36218

  • S0lo
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I have one instant clarification that is it possible to find exact subnet mask of any give ip address ie.. say IP Address is 172.19.20.100

No, as Nevins said. Except that you are able to find the CLASSFULL subnet mask of an IP address which can give you only a hint. For example:

IP: 160.100.1.1 is of class B, so the CLASSFULL subnet mask is /16. so the REAL subnet mask can NOT be less than /16. In other words /16, /17, /18, /19....etc are all OK. /15, /14... or less is NOT ok.

IP: 201.3.3.14 is of class C. so the CLASSFULL subnet mask is /24. so the REAL subnet mask can NOT be less than /24. In other words /24, /25, /26....etc are all OK. /23, /22... or less is NOT ok.

Sure you can defy this in an experimental private network, but thats not the standard.

Hope this helps.
Studying CCNP...

Ammar Muqaddas
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Re: is it possible 2 find Subnet Mask 5 years 7 months ago #36219

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Thanks,

i guess you haven't read my another post other than Top above, Would u mind provide any solution for that.

Ok let me paste that here again.

i would like to ask one question other than our normal procedure, since you said POINT TO POINT, will we able to use different Network ID or Different Class address in the point to point link.

1) Like Router 1 belongs to Head office and Router 2 Belongs to Branch office.

2) Head office in US and Branch office in India.

3) At this Point ISP Providers are not the same.

4) So assume Head office ISP U.S gives One Static IP Addrress 10.20.40.95

5) Branch Office ISP India gives One static IP Address 195.155.34.26

6) Now at any cost we must connect the both R1&R2 routers POINT TO POINT.

7)Would you mind shed some light on this.

Thanks in Advance,

Rizin
Known is a drop, unknown is an Ocean
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Re: is it possible 2 find Subnet Mask 5 years 7 months ago #36220

  • Nevins
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i would like to ask one question other than our normal procedure, since you said POINT TO POINT, will we able to use different Network ID or Different Class address in the point to point link.

1) Like Router 1 belongs to Head office and Router 2 Belongs to Branch office.
2) Head office in US and Branch office in India.


3) At this Point ISP Providers are not the same.

4) So assume Head office ISP U.S gives One Static IP Addrress 10.20.40.95
5) Branch Office ISP India gives One static IP Address 195.155.34.26


6) Now at any cost we must connect the both R1&R2 routers POINT TO POINT.

7)Would you mind shed some light on this.


You stated have 2 offices with 2 unique ISPs that gave out 2 unique IP addresses and physically they are on 2 ends of the globe and you must connect them with "POINT TO POINT" Links.

You asked will we able to use different Network ID or Different Class address in the point to point link?

The simple answer to your question is no. Completely ignoring your example a point to point link has to be on a single subnet. You can define a point to point link as a direct connection from one point to another over a single subnet. You will not find the addresses 10.20.40.95 and 195.155.34.26 on the same subnet therefore they cannot be connected by point to point links.
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