Mike Pennington
  • Member for 8 years, 6 months
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6 answers
128 votes
786k views
How do you calculate the prefix, network, subnet, and host numbers?
Accepted answer
197 votes

Calculating the Netmask Length (also called a prefix): Convert the dotted-decimal representation of the netmask to binary. Then, count the number of contiguous 1 bits, starting at the most ...

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1 answers
42 votes
5k views
Why is such a large IPv4 range assigned to localhost?
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46 votes

It comes from RFC 990, pg 5, under "Special Addresses": The class A network number 127 is assigned the "loopback" function, that is, a datagram sent by a higher level protocol to ...

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4 answers
22 votes
71k views
Why must all areas connect to an Area 0 backbone in OSPF?
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40 votes

OSPF Backbone Why is area 0 the backbone area in OSPF? Why must all other areas connect to it? This is explained very well in RFC 3509, Section 1.21: 1.2 Motivation In OSPF domains the ...

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1 answers
33 votes
79k views
Using SNMP to retrieve the ARP and mac-address tables from a switch
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39 votes

There seems to be a little confusion... you are asking about ARP tables, and you're using OID .1.3.6.1.2.1.17.4.3.1.2; however, that OID actually is for the mac-address table in the switch. I am ...

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4 answers
40 votes
24k views
How to check for a valid public IP address?
38 votes

Besides the original RFC 1918 space (which is now updated to RFC6890), there are several other blocks such as 192.0.2.0 that are not announced publicly. Furthermore, it's possible that someone has ...

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4 answers
40 votes
39k views
Bits per second vs. Packets per second
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32 votes

I just want to briefly mention the reality of marketing math when you're considering vendor data sheets. It is very common for vendors to double-count bps or pps capacity when you have full-duplex ...

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5 answers
32 votes
5k views
Why do we still use Ethernet?
28 votes

Since we use Ethernet mostly only as a means to transport IP, is there any reason to have that extra layer of L2 overhead? Naming a few common protocols or features that require L2 overhead such as ...

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5 answers
40 votes
37k views
Why was the MTU size for ethernet frames calculated as 1500 bytes?
28 votes

The answer is in draft-ietf-isis-ext-eth-01, Sections 3-5. Ethernet uses the same two bytes different ways in the Ethernet II (DIX) and 802.3 encapsulations: Ethernet II uses the first two bytes ...

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4 answers
23 votes
10k views
Does physical distance affect download speed?
26 votes

My colleague believed it's because of the physical distance while I don't think it matters. My understanding is once you have done the initial handshake and the data flow has started, it doesn't ...

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5 answers
23 votes
52k views
Why use iBGP inside an Autonomous System, if IGP protocols fulfill the need for internal communication
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26 votes

Can anyone explain me what is the need of IBGP communication for the routes, when we have the IGP protocols (OSPF, RIP) for internal communication? Scalability1: Imagine that you're receiving 500,000 ...

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6 answers
19 votes
10k views
Why use IP if we already have a MAC address?
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23 votes

Since Switch is a layer 2 device, and it uses MAC addresses to interact with other Hosts within the Network, then why do we use IP addresses within our local networks? Well let's start with what ...

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2 answers
11 votes
4k views
Why is RIP not scalable?
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21 votes

Most of the references say "RIP is not scalable" hence can be used only in smaller networks. But none say "WHY?" What is that in RIP that actually preventing it from scaling to ...

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2 answers
14 votes
10k views
ASICs vs x86 general-purpose routing/switching
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21 votes

Is it correct in assuming that ASICs for router or switch interfaces will outperform the use of an x86 CPU for all packet processing which will greatly suffer from CPU interrupts? It's hard to say ...

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3 answers
15 votes
22k views
MPLS vs Encrypted VPNs - traffic security?
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20 votes

Both Daniel and John gave very good answers to your question; I'll just add some practical things that come to mind when I read the question. Keep in mind that much discussion about the security of ...

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2 answers
19 votes
25k views
Ethernet autonegotiation differences between (10M | 100M | 1G) Ethernet
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20 votes

When autonegotiation fails on one node, to choose (half/full-duplex) we must use the rule: If you have a 10/100 Mb/s interface -> use half-duplex If you have a 1000 Mb/s interface-> use full-...

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1 answers
12 votes
26k views
What is the difference between Ethernet II and 802.3 Ethernet?
19 votes

802.3 (which uses 802.2 LLC format) has a Length field in the same place that Ethernet II has a Type field. IEEE 802.3 with 802.2 LLC (used by Spanning-Tree, ISIS) use the highlighted bytes for a ...

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1 answers
11 votes
1k views
Cisco IOS programming, automation, and information
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18 votes

Almost everyNote 1 Cisco IOS system running at least IOS 12.2 has built-in tcl shell programmability. You can store tcl scripts in flash and run them. Cisco IOS also has a light native programming ...

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2 answers
14 votes
10k views
What is a non-portable IPv4 address block?
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18 votes

Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a deeper, technical meaning? Non-portablility isn't about sales... it's a technical term. Address ...

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2 answers
14 votes
2k views
BGP regular expression .+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+
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18 votes

Every time you see .+_, that regular expression represents a single BGP autonomous system. This as-path list denies BGP as paths equal to or longer than 12 paths long. The regular expression works ...

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2 answers
11 votes
2k views
RIP routing is broken between two routers
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18 votes

they are assigned by default to 172.16.0.0 in both cases, and it does not work I modified your ascii art a litte to reduce scrolling... It sounds like you're saying that you can't reach reach N1 from ...

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3 answers
10 votes
19k views
What is the relationship between the bandwith on a wire and the frequency?
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17 votes

Modulation and symbols the number of occurrences of a repeating event per unit time. So what is repeating in the wire per unit time? The voltage patterns on the wire repeat. In extremely simple ...

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2 answers
12 votes
11k views
Error: `EIGRP is restricted to stub configurations only on this platform`
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17 votes

I really don't like how Cisco crippled EIGRP in the ipbase image... at a minimum, you should get some kind of warning that your configuration won't work, but they haven't done that either. As you ...

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3 answers
11 votes
1k views
Do today's routers prevent fake IP headers?
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17 votes

I know people can modify the IP headers and change source IP address, but it should be simple for network devices to detect those messages. Fake IP source addresses in headers can be detected and ...

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3 answers
7 votes
6k views
Edit Cisco IOS ACL without hanging the router
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16 votes

The reason your router hangs is because you're editing the ACL through the same interface that it's applied to. Usually what happens is you wind up blocking yourself accidentally. Note that I make a ...

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1 answers
12 votes
4k views
Point Of Presence vs. Central Office
Accepted answer
16 votes

What is the difference between this two (in distance and function). Short answer: Distances - not very much, these days Function - Basic / Enhanced 911 is still processed through the Central Office;...

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2 answers
16 votes
10k views
BGP multipath with different ASNs feasible for production networks?
16 votes

bgp bestpath as-path multipath-relax was introduced by CSCea19918. Normally eBGP load-balancing requires the candidate routes to be equal-cost paths; i.e. identical BGP attributes: same Weight same ...

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3 answers
14 votes
47k views
How to Monitor Vlan Traffic?
16 votes

Poll ifHCInUcastPkts, ifHCOutUcastPkts, ifHCInOctets and ifHCOutOctets on Vlan100, Vlan101, and Vlan102. You can find the ifIndex of these SVIs by walking ifName This get me another question : Why ...

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3 answers
10 votes
2k views
DHCP assign IP address based on MAC address with Cisco CE 500 platform?
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16 votes

Is it possible to allot IP address based on MAC address on CISCO CE 500 No you need a real Cisco IOS switch for DHCP server functionality with Maunal Bindings; the Catalyst Express series switches do ...

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1 answers
11 votes
3k views
SNMP v2: What is the difference between SNMP v2c and SNMP v2u
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15 votes

SNMPv2u is a user-based security model, as specified in RFC 1909 and RFC 1910. SNMPv2c is a common community-based security model. SNMP 2u offers per-user authentication, similar to SNMPv3. SNMPv2u ...

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2 answers
12 votes
14k views
Technical difference between a Collision and Broadcast Domain
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15 votes

Theory By convention, a collision domain is a contiguous wired and / or wireless half-duplex segment (typically using CSMA/CD), which is a subset of the subnet or vlan's broadcast domain. Collision ...

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