DHCP from my ISP and IP Classless in a Cisco Router

Hello all,

I am receiving an IP address from my ISP via DHCP of  166.149.248.252/30... my router rejects that address with the following error...   %IP-4-CLASS: Bad IP address and mask 166.149.248.252/30 in class_resolve()

I realize if you plug this into a Classfull calculator that the 252 address is the broadcast address for that network.  But I thought turning on IP Classless would allow you to do this.

I called my ISP,  Verizon, and they said it should work.

What am I missing?

Thanks
roedersAsked:
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TMekeelCommented:
252 is the 0 (network) address.
The subnet is 252-255.
1st useable should be 253 (your gateway) and your address should actually be 254, broadcast of 255.

The two hosts allowable on that subnet are 253 and 254 in other words.

What is the mask they are giving you?  Is it definitely 252?

DHCP is giving you 166.149.248.252 255.255.255.252?
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TMekeelCommented:
Im not sure why they are giving you DHCP on a point to point link anyway...
Verizon is usually a /24 in my experience unless it's a T1, which is usually statically assigned.
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roedersAuthor Commented:
Thanks for the comments,   this is a Celluar Router called a CradlePoint.  The Celluar service is provided by Verizon.  When you purchase the router they provide you with an IP and this is what I am getting via DHCP.   I am going to forward your comments to them.

Thanks
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rauenpcCommented:
I had this happen to me before, and actually it was with Verizon and a CradlePoint. If you plug the cell card into a laptop, you'll get the same ip/mask, but the laptop usually doesn't care. Routers, however, do care. I had to contact the account manager to submit a case to change my IP to a proper IP to resolve the issue.

IP Classless doesn't refer to tossing out all subnetting rules as Verizon is doing by sending you that ip/mask combo, but it allows routers to make decisions on a variable length subnet mask. Within each subnet we still need to follow the network/broadcast rules. The only exception that I know of is using a /31 address. Yes, you can configure a /31 on a Cisco device, but it must be a point to point type link. In this case the only two IP's possible are both network and broadcast, but they are treated as two host IP's instead. It's an oddball, but I've tested it because I thought I saw a typo in a Cisco document and had to see if I found a Cisco mistake.

R1#show ip int brie
Interface                  IP-Address      OK? Method Status                Prot                                                                                                             ocol
FastEthernet0/0            unassigned      YES unset  administratively down down                                                                                                             
Serial1/0                  unassigned      YES unset  administratively down down                                                                                                             
Serial1/1                  unassigned      YES unset  administratively down down                                                                                                             
Serial1/2                  unassigned      YES unset  administratively down down                                                                                                             
Serial1/3                  unassigned      YES unset  administratively down down                                                                                                             
R1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int fa0/0
R1(config-if)#ip address 192.168.0.0 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously
R1(config-if)#no shut
R1(config-if)#do show ip int brie
Interface                  IP-Address      OK? Method Status                Prot                                                                                                             ocol
FastEthernet0/0            192.168.0.0     YES manual up                    up                                                                                                               
Serial1/0                  unassigned      YES unset  administratively down down                                                                                                             
Serial1/1                  unassigned      YES unset  administratively down down                                                                                                             
Serial1/2                  unassigned      YES unset  administratively down down                                                                                                             
Serial1/3                  unassigned      YES unset  administratively down down                                                                                                             
R1(config-if)#
*Mar  1 00:01:30.383: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state t                                                                                                             o up
*Mar  1 00:01:31.383: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthern                                                                                                             et0/0, changed state to up
R1(config-if)#

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TMekeelCommented:
Rauenpc is as accurate as you can get there.  I think the /31 on a Cisco device needs ip subnet-zero turned on for that to work (I'd have to look up the actual command). In your case though, call Verizon and have them escalate it.
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roedersAuthor Commented:
Thank you all,  I have contacted Verizon and asked for a new IP address and forwarded your comments.
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roedersAuthor Commented:
Sorry,  I am a novice user of this service,  these points should certainly be split among all that answered my questions.

Thanks
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