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New Member

Dial-up connection and allocation of IP address / Subnet Mask / Default Gat

I have a dial-up connection thru telephone at my home..

Whenever I connect to the internet thru dial-up over telephone line, I get the following output for "ipconfig" and "ping" on the DOS Command prompt.

The command fired and output is as follows:

Windows 2000 IP Configuration

PPP adapter HCL InfiNet:

Connection-specific DNS Suffix . :

IP Address. . . . . . . . . . . . : 203.90.86.26

Subnet Mask . . . . . . . . . . . : 255.255.255.255

Default Gateway . . . . . . . . . : 203.90.86.26

C:\>ping 203.90.86.26

Pinging 203.90.86.26 with 32 bytes of data:

Reply from 203.90.86.26: bytes=32 time<10ms TTL=128

Reply from 203.90.86.26: bytes=32 time<10ms TTL=128

Reply from 203.90.86.26: bytes=32 time<10ms TTL=128

Reply from 203.90.86.26: bytes=32 time<10ms TTL=128

Ping statistics for 203.90.86.26:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\>

Now, my queries are:

1. Why does the output of "ipconfig" command show that both the dynamically allocated IP address of my computer and the Default Gateway's IP address are the same ? ( I understand that the default gateway's IP address is nothing but the Service Provider's router's IP address)

2. Why is the subnet mask 255.255.255.255 for the dynamically allocated IP address for my computer while it could have been the actual subnet mask of the IP address 203.90.86.26, like 255.255.255.240 ?

What is the reason for following the convention of giving a subnet mask of 255.255.255.255 for such dynamically allocated IP addresses over a dial-up link ?

3. In the output of the "ping" command, why does does the TTL appear as 128 ?

I understand that the default TTL of a IP packet is 255. I am pinging my own systems public IP address and it should return me a TTL value of 255 only, because it is not taking any hops.

In DOS, "ping" command can be fired with the TTL parameter manually changed and set anywhere between 1 and 255. I set the TTL as 255 once and 1 once and still the ping yields an output with TTL 128. Why is it so ?

Note: I did try out with such commands and outputs with dial-up conncetions from different Service Providers. The outputs were same.

5 REPLIES
Cisco Employee

Re: Dial-up connection and allocation of IP address / Subnet Mas

Replies

1)In PPP, "Point To Point", as per RFC, There is no way to negotiate default gateway ip address. so Winodows PC will by default assume that as the ip address negotiated. Because of "Point To Point" (PC to NAS), packets put on wire from PC will end up at NAS and NAS will do the rest of the routing. So PC don't really need default gateway as its own ip adddress can act as same.

As per PPP RFC, only ip address, DNS and WINS will be negotiated.

2)Again, its Point to Point (as name speaks), and since its not network based, the subnet mask for PPP link will he /32 always. Actual subnet mask will not be applied to PPP.

So PC and NAS both are the two end of PPP. So both will install /32 route for eachother.

3)128 is the windows PC's default TTL value. Its a function of Windows PC itself. Ping -i TTL should change the value but not sure why its can't. I am sure you will find some hits if you search www.microsoft.com

Re: Dial-up connection and allocation of IP address / Subnet Mas

That is correct and

Just to adding onto the point3

the reason ping -i TTL is unable to change the TTL value is because the PC you are pinging has already have tcpip loaded with TTL value of 128, which part of tcpip stack loaded on all windows machines. So the change does not get applied. Secondly there is no router in the middle, hence the changed value becomes void as the local loaded value takes priority.

Thanks

Silver

Re: Dial-up connection and allocation of IP address / Subnet Mas

Just an add on to point 3

If you need to change the default TTL value, you need to edit a registry key, see this link which talks more about that.

http://www.winguides.com/registry/display.php/885/

New Member

Re: Dial-up connection and allocation of IP address / Subnet Mas

With respect to our TTL discussion, I would like to add one more point from my observation.

When I tried pinging public IP addresses of hotmail or yahoo, the TTL used by the ping packets was 255, unlike a TTL of 128 which was used when I pinged my own public IP address which was dynamically negotiated over PPP for my dial-up connection.

So, does the TTL value set in a ping packet depend on the destination too ?

New Member

Re: Dial-up connection and allocation of IP address / Subnet Mas

1. But, then why should the default gateway's IP address also be negotiated

on a PPP ?

Is not the default gateway IP address a static IP ?

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