IP Addressing Refresher

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stsummers

New member
Dec 3, 2021
3
0
Greetings all. After the past couple of episodes of Security Now and Steve discussing the IETF plan on the 127 network space, I need a refresher. I took a network course 20 years ago and haven't worked directly with IP addresses in about 15. Can anyone recommend a good read to freshen up on IP addressing? Something free, preferably. If I had to explain how the slash notation worked at the end of addresses, I'd say, "Um....Hmmm..." a lot . Thanks.
 
I think the slashes things is newer than your training, which is probably why you don't really recognize it. It's called Classless Inter-Domain Routing (CIDR) as opposed to the old class based system where you had address classes https://www.meridianoutpost.com/resources/articles/IP-classes.php .

Basically when you're trying to decide if an IP address belongs on your network (and will be resolved by broadcasting to everyone to ask "who is this?") or it needs to be routed to another network, you need to break the IP address into two parts: the network part and the host part. You do this with a netmask, and the old system had predefined netmasks for Class A (255.0.0.0) Class B (255.255.0.0) and Class C (255.255.255.0). The 255's represent a byte with all bits set to 1. Where bits are set to one in the netmask indicates the part of the address that is the network part and where the bits are set to zero represent the parts that are the host part.

Class A which were all the IP's in the form n.x.x.x where n is between 1 and 126 (inclusive, assuming 0 and 127 is/was still reserved for localhost.) Class B was of the form n.m.x.x where n was 128 to 191. Class C was n.m.p.x where n was between 192 and 223. The Class D and E were special use cases like multicast. The truth of the matter is the first four bits of the address controlled most of the classes. 0xxx Class A; 10xx Class B; 110x Class C

For CIDR, you use a slash to indicate what number of bits are the network or host part. So 128.2.3.4/16 was an old Class B where half is network and half is host.

For CIDR, I googled and this was the first hit:
 
Last edited:
I think the slashes things is newer than your training, which is probably why you don't really recognize it. It's called Classless Inter-Domain Routing (CIDR) as opposed to the old class based system where you had address classes https://www.meridianoutpost.com/resources/articles/IP-classes.php .

Basically when you're trying to decide if an IP address belongs on your network (and will be resolved by broadcasting to everyone to ask "who is this?") or it needs to be routed to another network, you need to break the IP address into two parts: the network part and the host part. You do this with a netmask, and the old system had predefined netmasks for Class A (255.0.0.0) Class B (255.255.0.0) and Class C (255.255.255.0). The 255's represent a byte with all bits set to 1. Where bits are set to one in the netmask indicates the part of the address that is the network part and where the bits are set to zero represent the parts that are the host part.

Class A which were all the IP's in the form n.x.x.x where n is between 1 and 126 (inclusive, assuming 0 and 127 is/was still reserved for localhost.) Class B was of the form n.m.x.x where n was 128 to 191. Class C was n.m.p.x where n was between 192 and 223. The Class D and E were special use cases like multicast. The truth of the matter is the first four bits of the address controlled most of the classes. 0xxx Class A; 10xx Class B; 110x Class C

For CIDR, you use a slash to indicate what number of bits are the network or host part. So 128.2.3.4/16 was an old Class B where half is network and half is host.

For CIDR, I googled and this was the first hit:
Hey that's great! Yeah, showing my age I guess by asking my question. I'll save both the links and study up on them. I appreciate the clarification. :)