Basic Internet Connectivity And Modem Help

[ChasNH]

The IP address assigned to me is 24.147.77.255. That is NOT a valid HOST address, it is a broadcast address. (All IPs ending with 255 are broadcast.) That is why I can not reach some web sites - routers block the IP address. When will you fix your DHCP servers so that they assign in the proper IP range from x.x.x.1 to x.x.x.254? I want a reply to this ASAP. If you do not understand it, elevate it.

[Me]

What did Comcast say when you called it in at 800-comcast??

[ChasNH]

They said 1) That it was OK to have .255 at the end of the IP address, 2) It's not their fault if some ISPs block IPs ending with 255, 3) Nobody else complains, and 4) My address would change in a few days anyway. I told the service person that I wanted the issue elevated. He argued with me about that. He said that by the time someone looked at it that my IP address would be changed. He also said they would not get back to me.

This really inspires confidence. He says he will elevate it but I will receive no confirmation that he has elevated it.

My guess is that nothing will happen, and that I, and 1 out of 255 customers, will continue to have trouble accessing some web sites. BTW, the specific problem I encountered prevents me from reaching most universities and large companies in Quebec because it is RISQ who blocks .255 in their routers.

[Xoanan]

> The IP address assigned to me is 24.147.77.255. That
> is NOT a valid HOST address, it is a broadcast
> address. (All IPs ending with 255 are broadcast.)
> That is why I can not reach some web sites - routers
> block the IP address. When will you fix your DHCP
> servers so that they assign in the proper IP range
> from x.x.x.1 to x.x.x.254? I want a reply to this
> ASAP. If you do not understand it, elevate it.


Thats rather disquieting; dont most Comcast ips start with 68 or 69 regardless of what they end with? Thats the way it is here.

[Me]

Charlie...have you tried to "force" an IP change??

[Baric]

Can I ask how you know this is your IP address? You're right, this is invalid, but I find it hard to beleive that Comcast's DHCP server is assigning it, and even more surprised that your system is accepting it.

[ChasNH]

Yes, but that is not the issue. The issue is that x.x.x.255 is NOT a valid host address and should never be assigned to a host. RFC 1519 specifically says "class C, with a maximum of 254 host addresses", meaning x.x.x.1 through x.x.x.254. The reason the .255 works most of the time is that RFC 1812 specifically requires that routers must treat " , -1 " as a broadcast packet. However, because of the severity of Internet abuse, many ISPs violate RFCs by tightening up security. Further, there is no good reason to allow broadcast packets onto one's network. In using .255 for a host address, Comcast is just lucky that more ISPs are not doing the blocking.

[ChasNH]

The easiest way for a non-technical person to find one's IP address is to send an email to yourself and then examine the headers. Look above the "To" header to the header that says "Received:". Their should be an IP address enclosed in brackets.

[Jason1]

Charlie,

We have escalated this issue to our network team to investigate.

[ChasNH]

Thank you jason. See RFC 1812 5.3.5 which specifically says:

"A network-prefix-directed broadcast is composed of the network prefix of the IP address with a local part of all-ones or { , -1 }. For example, a Class A net broadcast address is net.255.255.255, a Class B net broadcast address is net.net.255.255 and a Class C net broadcast address is net.net.net.255 where net is a byte of the network address."

THEREFORE, x.x.x.255 is defined by RFC as a network-prefix-directed broadcast. The same RFC says:

"A router MUST classify as network-prefix-directed broadcasts all valid, directed broadcasts destined for a remote network or an attached nonsubnetted network. Note that in view of CIDR, such appear to be host addresses within the network prefix; we preclude inspection of the host part of such network prefixes. Given a route and no overriding policy, then, a router MUST forward network-prefix-directed broadcasts. Network-Prefix-Directed broadcasts MAY be sent.

A router MAY have an option to disable receiving network-prefix- directed broadcasts on an interface and MUST have an option to disable forwarding network-prefix-directed broadcasts. These options MUST default to permit receiving and forwarding network-prefix- directed broadcasts.

DISCUSSION
There has been some debate about forwarding or not forwarding directed broadcasts. In this memo we have made the forwarding decision depend on the router's knowledge of the destination network prefix. Routers cannot determine that a message is unicast or directed broadcast apart from this knowledge. The decision to forward or not forward the message is by definition only possible in the last hop router."

Please note that I am just trying to get resolution to a problem that affects all Comcast customers. I do not intend to be a nuisance.

[Baric]

Just use "ipconfig /all" in a Command prompt, I'm really curious what that says. This is tres strange.

[Barmar]

> Yes, but that is not the issue. The issue is that
> x.x.x.255 is NOT a valid host address and should
> never be assigned to a host. RFC 1519 specifically
> says "class C, with a maximum of 254 host addresses",
> meaning x.x.x.1 through x.x.x.254.

But 24.x.x.x is not a Class C address block, it's a Class A. The class A broadcast address is x.255.255.255.

But that's actually irrelevant these days, as address classes are obsolete. The only way to tell if an address is a broadcast address is if you know the network mask of that subnet. I believe Comcast uses 255.255.252.0 as their network mask in most areas, so the broadcast address on your segment would be 24.147.79.255. The valid addresses range from 24.147.76.1 to 24.147.79.254.

Anyone who is assuming that all x.x.x.255 addresses are broadcasts is misguided. Very few organizations have class C networks these days.

[Baric]

Comcast uses a number of different netmasks these days, I guess it depends on the number of customers they have in a given area. My IP have always started with 24.* or 68.* and my netmask used to be 255.255.255.0, but some time last year it changed to 255.255.248.0. So unless we know what Charlie's netmask is we don't know what the broadcast address is for his connection. I was simply taking him at his word , in the absence of any ipconfig /all output.

[Michael]

Barmar has hit the nail on the head. We are not using Class C address blocks. In a Class A block with variable subnetting, this address would be valid.

[f1ip]

CharlieNH: "The easiest way for a non-technical person to find one's IP address is to send an email to yourself and then examine the headers. Look above the "To" header to the header that says "Received:". Their should be an IP address enclosed in brackets "

you expect a NON-Technical person to know how to read headers in email? lmfao!!.. let alone how to view the headers.


how about just going to a simple website.. like.. www.whatismyip.com ??
Message was edited by: f1ip

[Baric]

24.147.77.255 is the broadcast address for any of hte following netmasks (unless I borked the math up):

255.255.252.0
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
255.255.255.248
255.255.255.252

The last few are just trivial examples of valid netmaks, but they have no practical application with Comcast since the rang of IP's is too small to be useful in a given area.

[Barmar]

> start with 68 or 69 regardless of what they end with?
> Thats the way it is here.

As a result of all the mergers and acquisitions, Comcast has access to quite a few address ranges. Here's the WHOIS record for this address block.

CustName: Comcast Cable Communications Holdings, Inc.
Address: 1800 Bishops Gate Boulevard
City: Mount Laurel
StateProv: NJ
PostalCode: 08054
Country: US
RegDate: 2005-04-27
Updated: 2005-04-27

NetRange: 24.147.0.0 - 24.147.255.255
CIDR: 24.147.0.0/16
NetName: BOSTON-3
NetHandle: NET-24-147-0-0-2
Parent: NET-24-147-0-0-1
NetType: Reassigned
Comment:
RegDate: 2005-04-27
Updated: 2005-04-27

[joseph_chapman]

Thankyou!
I was digging down through the responses and wondering how far I would have to go to see an intelligent, educated, response.

People are too quick to think they know what the problem is and stating solutions that aren't being implemented by Comcast instead of just stating the problem/symptoms.

Unfortunately, there's too many sources of bad information out there that people read and become self-certified 'experts'.