40 votes
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Why is 10.1.255.255 an invalid broadcast address?

I believe the book wrongly assumes network classes are still in effect. So a) would be a "Class A" network, where 10.255.255.255 would be the broadcast address. Another hint: There is no explicit ...
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26 votes
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WHY and HOW do routers and vlans break broadcast domains?

Let's talk about it using this topology of three networks (red / orange / blue): A Router's primary function is to facilitate communication between IP networks. Which means if A wants to speak to D ...
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How broadcasting works on different networks?

How Routers Handle Limited and Directed Broadcasts The first thing to understand to answer your questions is that limited broadcast frames are not routed. By default when a router receives a frame ...
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15 votes
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How do VLANs affect the broadcast domain?

Each VLAN creates its own broadcast domain in 1 or more physical switches. If you have a switch taken out of the box it tends to put all ports in vlan 1 so for say a 24 ports the broadcast domain ...
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  • 1,357
15 votes
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How does DHCP assign an IP address?

The basic process is quite simple. I'll only cover that and omit scenarios where several DHCP servers exist, error conditions crop up or discovery has to cross network boundaries. A new client on a ...
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13 votes
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pinging broadcast address

Not all machines will answer a broadcast ping. (all broadcast -- 255.255.255.255, or subnet broadcast -- eg. x.x.x.255) Some see it as a "security feature", because one could spoof the origin to flood ...
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11 votes
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What is the difference between broadcasting and flooding?

Broadcasting is a term that is used on a broadcast domain, which is bounded by layer-3 (routers). Broadcasts are sent to a special broadcast address, both for layer-2 and layer-3. A broadcast cannot ...
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11 votes
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Why couldn't a nework address be used as as broadcast address as well?

What is the point of reserving two IP addresses in a subnet when one might suffice? [...] If it's theoretically possible, why is it avoided? We live with our history. The distinction between address ...
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11 votes
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How is the Groupwise Transient Key used in WiFi networks?

Why then does the client need to encrypt the broadcast using the GTK? It doesn't. Since the AP broadcasts, not the client, the client doesn't use the GTK to encrypt the frame. The AP does. Why can'...
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8 votes

What are real life examples of broadcast and multicast?

Broadcasts are stopped at a layer-3 boundary (router). An example of a broadcast is an ARP request where a host is looking for the MAC address of the owner of an IP address. The host sends a broadcast ...
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8 votes
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What is the difference between layer-2 and layer-3 broadcasts? Please explain with an example

You need to understand the difference between layer-2 and layer-3 for an answer to have any real meaning for you, and I suspect you don't. A layer-2 network is a LAN, and all hosts on it are peers. A ...
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8 votes
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When and where to use Spanning-Tree (Portfast)

Spanning tree only runs between switches, never to standard end hosts. Bear in mind that, under normal circumstances, a port supporting spanning tree will (when it first comes up) run through a ...
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7 votes
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How does router prevent Broadcast radiation

But I do not understand how exactly routers resolves broadcast storm problem. Any explanation in detail? When a router receives a packet, it gets inspected, then forwarded out the appropriate ...
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7 votes

Why do we use VLAN?

Besides the unknown unicast flooding that you allude to, broadcasts are necessary to the operation of ethernet. Some protocols require broadcasts. For instance, hosts use ARP requests (broadcasts) to ...
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7 votes

What is the difference between broadcasting and flooding?

I will explain here in the context of switches(Layer 2) FLOODING : When a switch receives a unicast frame(a frame with a specific mac address intended for a particular device), it looks for the frame'...
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Do routers forward broadcast traffic?

IPv4 uses limited broadcasts (to 255.255.255.255) and directed broadcasts (to the subnet address with all host bits set to 1, e.g. 10.0.0.255 for the 10.0.0.0/24 subnet). Limited broadcasts are ...
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Why is 10.1.255.255 an invalid broadcast address?

This is a "traditional" exam. question which contains: Missing information A trap A hint The missing information is the subnet mask or CIDR number of bits. The trap is answer (a) : 10.1.255.255/16 ...
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  • 340
6 votes

Identify gratuitous ARP

You can identify Gratuitous ARPs by looking at the ARP Sender Protocol Address and ARP Target Protocol Address, so as you mentioned when they are the same it's a gratuitous ARP. See RFC 2002, Section ...
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Must a broadcast request come from the same network?

Broadcasts do not normally cross layer-3 (routers). Some routers can be configured to forward subnet broadcasts, but the limited broadcast (255.255.255.255) will not cross layer-3. The Cisco version ...
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6 votes

Why do we use VLAN?

There can be many reasons to split a network into VLANs. Security seggregation. You may want to filter or forbid communication between some systems on your networks. Limiting of broadcast (and ...
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6 votes
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Is the real broadcasting on internet possible

I don't think you really understand that the Internet is just a collection of many ISPs which connect to other ISPs. Each ISP has its own policies. Also, IP has a single source and a single ...
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6 votes
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Does a switch MAC address table sync across all switches in the same VLAN?

Synchronizing MAC address tables across switches doesn't make any sense. Each switch maintains its own MAC address table. For instance, suppose you have Switch 1 and Switch 2 connected together of ...
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6 votes
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Broadcasting traffic by 255.255.255.255 or by "subnet dot all ones"?

An application can send broadcasts to either the network broadcast address (network all ones) or the limited broadcast address (all ones address). It used to be the default for routers to allow ...
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6 votes
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Why does OSPF use flooding instead of broadcasting?

You are confusing the layer-2 flooding with OSPF flooding. The routers in an OSPF area may not all be directly connected, but every router in the area needs to have a full understanding of all the ...
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6 votes

Can a large broadcast domain break accesspoints

First, please forget about classfull networks, they are dead for decades now. Having a /18 network is not recommended at all. As you pointed out, it is a very large broadcast domain that will cripple ...
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6 votes
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Does a dial-up modem support multiple users?

Almost all consumer Internet access links use only a single IP address and many business links as well. A single IP address can't be shared directly but extremely commonly (source) NAT is used. A ...
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6 votes

Is Ethernet a broadcast or point-to-point protocol?

Initially, Ethernet was based on broadcasting physical layer technology (bus network for 10BASE5/10BASE2 or with repeaters), but logically (layer 2), the protocol has always been MAC-based many-to-...
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6 votes
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Why are DHCP ACK and REQ messages broadcast in Link Layer?

From RFC 2131 "Dynamic Host Configuration Protocol" Remembering that there can be multiple DHCP servers, the DHCPREQUEST is broadcast because: Those servers not selected by the DHCPREQUEST ...
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6 votes
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Flooding vs Unknown Unicast Flooding

As @ronmaupin points out, "flooding" is a generic term for sending data everywhere (usually except the source). OSPF multicasts link state advertisements, and we colloquially say OSPF floods them in ...
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5 votes

Arping for local broadcast address

Given the local IP-network 10.173.128.0/24 I am looking at a wireshark and there is an ARP for 10.173.128.255. Why would a network client ever ARP for a broadcast address? A ...
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