How Are IP Addresses and Subnet Masks Related?

One of the topics that I (along with many others) had trouble understanding was how to differentiate the network side of an IP address with the host side of an IP address. For example, our computer has an IP address 192.168.1.1 with a subnet mask of 255.255.255.0. The subnet mask is key in determining the difference between your network address and host address.

Now let’s break down our computer’s IP. Every IP consists of four, 8-bit octets that range in decimal value from 0 to 255. For instance, 192 (our IP’s 1st octet) in decimal translates to 11000000 in binary (for more information on binary to decimal translation, see this article: http://www.wikihow.com/Convert-from-Binary-to-Decimal). This next step is key. ANY BINARY ‘1’ IN OUR SUBNET DEFINES THE NETWORK, and ANY ‘0’ IN OUR SUBNET DEFINES OUR HOST. Because our subnet mask’s 1ST, 2nd, and 3rd octets are 255 (or 11111111 in binary), this means that our network address that our IP exists in is 192.168.1.0. The 4th octet, thus, defines the host number in this network. So in this one particular network, 192.168.1.0, we can have up to 254 host computers. You may wonder why not 255?? This is because the host address 255 (192.168.1.255) is reserved for the network’s broadcast address. Therefore, we can only host 254 addresses on our network (1-254).

Let’s try a little harder example this time. Say we go to the command line on our computer and do an “ipconfig” command to display our IP and subnet mask. The output displays our IP address being 192.168.1.193 with a subnet mask of 255.255.255.192. The first thing to do is break the IP address into each octet. Luckily for us, the 1st three octets are all our network address as defined by the subnet being all 1’s (255 = 11111111 in binary). Now all we need to worry about is our last octet! The 192 in our subnet mask translates to 11000000 in binary (All ‘0s’ being possible host IP addresses). Because our network is defined by 1’s in the subnet, the first 2 bits of the last octet of our IP are still part of the network address. So, if we translate 193 to binary, we get 11000001. 193 ends up being the first host in the network 192.168.1.192! So in this case, our network address is 192.168.1.192 in which hosts in this network will range from 192.168.1.193 – 192.168.1.254!

Fortunately for us, IPv6 has been created which voids the need for differentiating the network address from the host address using a subnet mask. I will post this topic in a later article, but for now, IPv4 takes practice, practice, and even more practice to understand the concept of the relationship between your IP address and subnet mask. Try it yourself on your computer at home and let me know how things go!

IP Multicast Technology Overview

Most of us are very familiar with p2p networks concept. They help to share or download files from many different sources at the same time, some of them are determining the closest source location before the download process starts. Even though the IP multicast works a little bit different than that, the concept and the goal is the same: to optimize the traffic and to utilize network connection more efficient if possible.

IP communication allows a host to send packets in two manner:

1. To a single host (called unicast transmission)
2. To all hosts (called broadcast transmission)

IP multicast provides a third possibility:

3. To a subset of all hosts (called a group transmission)



IP multicast is a bandwidth conserving technology that redueces traffic by simultaneously delivering a single stream of information to potentially thousands of corporate recipients and homes. Applications that take advantage of multicast include video conferencing, corporate communications, distance learning, and distribution of software, stock quotes, and news.

IP multicast delivers application source traffic to multiple receivers without burdening the source or the receivers while using a minimum of network bandwidth. Multicast packets are replicated in the network at the point where paths diverge by Cisco routers enabled with Protocol Independent Multicast (PIM) and other supporting multicast protocols, resulting in the most efficient delivery of data to multiple receivers.

Many alternatives to IP multicast require the source to send more than one copy of the data. Some, such as application-level multicast, require the source to send an individual copy to each receiver. Even low-bandwidth applications can benefit from using Cisco IP multicast when there are thousands of receivers. High-bandwidth applications, such as MPEG video, may require a large portion of the available network bandwidth for a single stream. In these applications, IP multicast is the only way to send to more than one receiver simultaneously.

IP multicast addresses specify a “set” of IP hosts that have joined a group and are interested in receiving multicast traffic designated for that particular group. The Internet Assigned Numbers Authority (IANA) controls the assignment of IP multicast addresses. IANA has assigned the IPv4 Class D address space to be used for IP multicast. Therefore, all IP multicast group addresses fall in the range from 224.0.0.0 through 239.255.255.255.

The most important terms are:
RP (Rendezvous Point) - it is designated router in your network that is usually the "center" of it. It receives and decides which path to choose to deliver packet to all receivers that are interested.

PIM (Protocol Independent Multicast) - is IP routing protocol-independent and can leverage whichever unicast routing protocols are used to populate the unicast routing table, including Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), Border Gateway Protocol (BGP), and static routes. PIM uses this unicast routing information to perform the multicast forwarding function. Although PIM is called a multicast routing protocol, it actually uses the unicast routing table to perform the RPF check function instead of building up a completely independent multicast routing table. Unlike other routing protocols, PIM does not send and receive routing updates between routers.

PIM-DM (PIM Dense Mode) - uses a push model to flood multicast traffic to every corner of the network. This push model is a brute force method for delivering data to the receivers. This method would be efficient in certain deployments in which there are active receivers on every subnet in the network.

PIM-SM (PIM Sparse Mode) - uses a pull model to deliver multicast traffic. Only network segments with active receivers that
have explicitly requested the data will receive the traffic.

Bidir-PIM (Bidirectional PIM) - is an enhancement of the PIM protocol that was designed for efficient many-to-many communications within an individual PIM domain. Multicast groups in bidirectional mode can scale to an arbitrary number of sources with only a minimal amount of additional overhead.


More information visit a Cisco documentation page, for a configuration guides click here.

Etherchannel in a Nutshell: Understanding and Configuring the Cisco Technology

Ever feel like your internet connection isn’t as fast as you think it could be? Along with that, shouldn’t there be a way to create fault tolerance between your computers and the internet in the case that one of your cables happens to go bad? Fortunately for you, there is a way to kill two birds with one stone. It’s called EtherChannel, an easily-configurable technology used primarily on Cisco switches.

Etherchannel allows the grouping of multiple, physical Ethernet links into one logical link. This provides both increased bandwidth as well as fault tolerance between your routers, switches, servers, hosts, etc. Each Etherchannel can consist of between two and eight Fast Ethernet, Gigabit, or 10 Gigabit Ethernet channels. This means that, depending on how many Ethernet links you create, you can create multiplied bandwidth as well as fault tolerance without losing connection on your newly created, grouped Ethernet link.

For example, let’s say you have two Cisco switches, both which have four Gigabit Ethernet ports. You have already established connectivity using one Gigabit port on each switch, but users are saying it is taking them too long to transport large, necessary files to each other. Each of our switches has 3 additional Gigabit Ethernet ports, but unfortunately all they are doing right now is collecting dust. Creating 3 additional physical Gigabit Ethernet links allow us to group these all together into one logical link using Etherchannel to multiple our original Gigabit speed by four AND create backup links at the same time in case one of them fails! So in essence, our previous bandwidth of 1 Gbps is now 4 Gbps including fault tolerance!!!

For additional information on the subject, please refer to the following links:

Cisco EtherChannel Technology

Configuring EtherChannel

Using Question Mark For Secret Password With Cisco Routers and Switches

As you know a question mark in Cisco IOS (Internetwork Operating System) is used to display available options in the context you actually are. What if you want to use a question mark as a one of the letters of your password? We will assume the password you want to use is "qm?".

If you use it here is what will show:

Router(config)#enable secret ?
0 Specifies an UNENCRYPTED password will follow
5 Specifies an ENCRYPTED secret will follow
LINE The UNENCRYPTED (cleartext) 'enable' secret
level Set exec level password

Even if you type the beginning of it:

Router(config)#enable secret qm?
LINE
Router(config)#enable secret qm

It still comes back to just "qm". So how can we use it? You need to press CTRL+v prior to pressing "?":

Router(config)#enable secret qm?

Whoala! The magic combination was CTRL+v.
By the way you may have the idea to cut and paste a "?" from the notepad into terminal try it... and let us know.