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15
votes
3answers
6k views

Configure Cisco ASA in Transparent mode: Layer2 DMZ w/ Vlan translation

I am in the middle of a project to migrate some existing switched ethernet dot1q trunks behind an ASA firewall... these trunks have five vlans each (numbered 51 - 55). This is a simple drawing of the ...
12
votes
1answer
4k views

Mac / CAM entries from non-Cisco switches with Q-BRIDGE-MIB

How do I do community string Indexing for other switches like D link DES 3526 and Brocade Fastiron SX800? I am running a java program to poll a specific vlan on a Brocade sx 800 l3 switch. The ...
8
votes
1answer
2k views

UDP hole punching NAT port

I just read an article about the process of "punching holes" with the UDP protocol. I understand how it's done, but I have one last question to clear up things. We have the rendezvous server and ...
6
votes
2answers
2k views

What is the reason for the different order of the Source and Destination in a L2 header?

A TCP/UDP header (Layer 4) lists the Source Port, then the Destination Port: An IP header (Layer 3) lists the Source IP, then the Destination IP: An Ethernet header (L2) and a WiFi Header (L2) lists ...
2
votes
3answers
423 views

Why can't a host B act as a server, and accept TCP connections, when sitting behind a NAT?

Originally, I thought I intuitively understood why B can't accept TCP connections, however when I read about how P2P applications circumvent this exact problem by using an intermediate peer that is ...
19
votes
2answers
24k views

Ethernet autonegotiation differences between (10M | 100M | 1G) Ethernet

I am studying for CCNA and on Wendell Odom's book is said that(regarding autonegotiation): When autonegotiation fails on one node, to choose (half/full-duplex) we must use the rule: If you have a 10/...
10
votes
2answers
4k views

KEMP load balancers using UCARP (VRRP) - multicast MAC address not being picked up

Alright - been battling this for at least 20 hours consecutive.. Sorry if this seems like a long rant, or a blog post, but I'm getting to the point of exhaustion. So, here's the deal. We're using ...
6
votes
2answers
573 views

VLAN-Subnets 1:1?

I often hear people saying that each VLAN is a subnet (or is associated with a subnet) but is that really true? Assume I have 10 big offices. I will have a subnet for each office but only 2 VLANs ...
4
votes
4answers
2k views

Can IP packet with TTL=1 reach loopback interface of a switch?

This is a question that bothers me a lot. And I cannot argue with colleagues because all switches I see can response the ping request even my TTL=1. So my question is, does TTL decrease between ...
3
votes
1answer
541 views

How does a router know to which PC in a network to send a response from a server?

When my PC sends a request to a server, the transmitted IP address is the one of my private network. When the server responds to the request, how does the router know to which PC in the network to ...
3
votes
1answer
1k views

Can a 172.168.10.0/28 be a class B Subnet

I am starting my studies in networking and starting off with the basics of IP addressing and Subnetting with CIDR values. My Question here is 1st how can you tell the subnet class of an IP address ...
2
votes
3answers
220 views

Is an entirely decentralised peer-to-peer network feasible? How far could it be scaled?

Most of the reading I've done about P2P networks are either about small scale simple mesh networks, where every peer can talk to another, or about high level protocols such as bittorrent, where data ...
34
votes
2answers
89k views

What is the actual size of an Ethernet MTU

I think I might be getting confused with terminology surrounding MTU. This definition from Wendell Odom's CCNA book on MTU: The IEEE 802.3 specification limits the data portion of the 802.3 frame ...
12
votes
3answers
40k views

How does a switch learn a MAC address not in its lookup table?

I have a relatively dumb question. Suppose the Switch just started, and it received a frame that contains a destination MAC address for a network device not in its MAC addresses table. What happens ...
37
votes
3answers
109k views

What is the difference between MSS and MTU?

I'm confused as to the difference between maximum segment size and a maximum transmission unit. Can someone please explain in relation to layers 2 and 3? If I had a packet of 800 bytes in the ...
17
votes
3answers
69k views

Is the “default VLAN” simply the default native (untagged) VLAN on all interfaces that have no configuration?

... or does the "default VLAN" have some wider meaning? Also, can it / should it be changed? For instance, if a switch is going into part of a network that is only one VLAN and it's not VLAN 1, is ...
12
votes
1answer
24k views

What is the difference between Ethernet II and 802.3 Ethernet?

Can anyone explain to me what is the difference between Ethernet, Ethernet-II and 802.3 Ethernet. I referred to a lot of books, they are in high level language. Can anyone explain to me in a simple ...
18
votes
4answers
23k views

Why does WLAN use Collision Avoidance and not Collison detection?

I am searching for this answer for quite some time now. Why there is CSMA/CD in LAN but CSMA/CA in WLAN? The best explanation I could find is "due to extreme ratio in transmitting and receiving power,...
7
votes
3answers
5k views

Does a switch understand packets? If yes, what is the frame terminology for?

I was a little confused between packet and frame. The confusion was based on, if a switch is a layer 2 device, how does it reads a packet from a layer 3 device, and where is the 'frame' thing which is ...
15
votes
6answers
26k views

How are IP addresses mapped to MAC addresses?

I've been reading about the differences between MAC and IP addresses, and why we need both of them. To summarize, MAC addresses are physical unchangeable unique IDs for every single device while IP ...
18
votes
4answers
20k views

What's the difference between `Latency` and `Round Trip Time`?

Golang community provide a HTTP/2 demo website to compare the performance between HTTP 1.1 and HTTP/2. We can choose different latency, e.g. 0s latency, 30 ms latency, 200ms latency. Is latency a ...
22
votes
3answers
10k views

Exactly when is PMTUD performed? (Path MTU discovery)

In discussions that have spurred from other questions on this site, I've realised that I don't have a solid understanding of when Path MTU Discovery (PMTUD) is performed. I know what it does -- ...
19
votes
1answer
5k views

What is the practical use of a /31 CIDR or 255.255.255.254?

With a /31 subnet, we are left with zero usable hosts. The /31 subnet only has two hosts - one for network and another for broadcast. I am wondering why would someone ever use this subnet.
13
votes
4answers
5k views

/31 Point-to-point bitmasks

When is it appropriate to use a /31 network in production, and is using them considered good practice? On a point-to-point link, broadcasts shouldn't be required, so is there any compelling case for ...
13
votes
5answers
12k views

When is BGP needed?

I have a question about BGP. When exactly is BGP needed? I remember reading that BGP is needed for communication between autonomous systems, but is this always true? For example, in packet tracer I ...
12
votes
3answers
2k views

Is there such thing as a network bridge?

Is there such a thing as a network bridge or is it always meant as a switch implementing a bridging function? Whilst bridges are described as devices that create separate collision domains in network ...
7
votes
1answer
2k views

Why is the NDP Neighbor Solicitation message sent to the solicited-node address?

As I understand it, when Host Alice knows Host Bob's IPv6 address but not its MAC, Alice sends an NDP Neighbor Solicitation (NS) message to the solicited-node multicast address for Bob. Alice knows ...
15
votes
2answers
9k views

In which OSI/TCP-IP model layers do BGP, RIP protocols belong?

We know that in the OSI or TCP-IP model, the higher layer utilizes the services offered by the lower layer. For example, the data link layer uses services offered by the physical layer, network layer ...
14
votes
2answers
12k views

Why does “show interface” show “Last input never” when packets input is incrementing?

Why is it that when I perform a show interface command I see that Last input always shows never? FastEthernet1/31 is up, line protocol is up (connected) Hardware is C6k 100Mb 802.3, address is 000d....
11
votes
1answer
363 views

Is a FIN only segment legal?

It would be convenient to mark TCP segments with just the FIN flag set, as an intrusion (without tracking the reply). I have always assumed that a FIN without an ACK, while rude and rare, is legal, ...
9
votes
3answers
1k views

What is a “Layer 3 Switch”?

What is a "Layer 3 Switch"? I essentially understand the OSI model, well maybe I don't and maybe that is why I don't understand. Its just that I don't see how/when/if a "managed switch" becomes a "...
9
votes
4answers
1k views

What Exactly is SDN?

Is SDN a buzzword or is it a protocol, standard, or recommendation? What differentiates SDN from VMware NSX, for example?
8
votes
1answer
2k views

Staging Many Lightweight Cisco AP's for Deployment

My organization is soon going to be deploying approximately 600 new Cisco 2602i access points in a CAPWAP deployment with several 5508 controllers. This is an infrastructure refresh; currently we ...
8
votes
7answers
20k views

Juniper EX-series: How can I transfer a config from one switch to another?

I have a lab EX-2200 that has a config on it that I now want to replicate onto four more EX-2200's... I'm sure there's probably an easy way to do this, but couldn't find it on the Juniper KB or by (a ...
3
votes
1answer
3k views

Traceroute UDP port question

I know, that traceroute uses UDP, and for destination port it starts with 33434. Then, with each next hop it increments the destination port number by 1. The questions is: why does traceroute need to ...
3
votes
1answer
2k views

minimum hop limit for a packet where there are n routers between source and destination

I am having some confusion while calculating he minimum hop limit ... consider following scenario: (source)-------(R1)--------(R2)--------(destination) Now when an IP datagram is supposed to travel ...
1
vote
2answers
434 views

Why do transport layer do data chunking. If there is fragmentation in Network Layer

I just curiously wanted to know about the question regarding L4, L3 Data Chunks, IP Fragmentation : Data chunking happened at Layer 4(Transport, TCP), and Fragmentation at Layer 3(Network, ipv4) ...
14
votes
4answers
1k views

Is a wireless only network (for end users) a viable option?

We are constructing a new building, and our CIO is pushing for a solution where he doesn't have to spend the money on copper runs to all the user stations and wants to use wireless only for this ...
13
votes
1answer
1k views

What happened to IPv5?

I hear/read much about IPv4 and IPv6 and nothing about IPv5. Where did it get lost in the discussion?
12
votes
7answers
14k views

DHCP relay problem on interfaces secondary network

A secondary network was configured on an interface of a Cisco 2621 router and the ip helper-address commands put in place but no clients of that secondary network could pull DHCP addresses. We swapped ...
11
votes
1answer
10k views

How is BGP “AS_SET” used?

RFC 1771 defines a path attribute type of AS_PATH as follows: AS_PATH (Type Code 2): AS_PATH is a well-known mandatory attribute that is composed of a sequence of AS path segments. Each AS path ...
10
votes
1answer
3k views

How does NAT share one public IPv4 address across multiple private IPv4 addresses [closed]

I think it's a very classical question, but, thought I've have some background, I don't have enough vocabulary anymore to search and understand correctly in the Web. Let's say I have a home network ...
10
votes
3answers
4k views

Inbound BGP load-balancing from same ISP router

I have two BGP routers for DIA with an issue with one provider in that the two links to this provider land on the same SP router. My ethernet handoffs via two separate MetroE links from another ...
9
votes
1answer
13k views

ProCurve CPU at 100% for several minutes after reboot

I have been doing firmware upgrades in HP switches. Two different models have been upgraded: ProCurve Switch 5406zl Intelligent Edge (J8697A): upgraded from K.15.06.0008 to K.15.12.0012 HP 2520-24G-...
9
votes
4answers
3k views

Switch interface config for IP Phones

I have seen both of the following methods of switch interface configuration for supporting IP Phones. switchport access vlan <data-vlan> switchport voice vlan <voice-vlan> switchport ...
7
votes
3answers
4k views

How does NAT decide which connections are inbound, and which are outbound?

I'm building a P2P application that needs to be able to access nodes behind a NAT. This NAT does not allow inbound connections, therefore the nodes outside the network cannot reach out to the nodes ...
6
votes
1answer
513 views

Why do we have authentication before association in 802.11

Why does client authenticate before associating with the AP. What does open system authentication even achieve. I fail get an accurate reason from my web searches. Best reason I do get is unlike wired ...
6
votes
1answer
6k views

ARP request unicast MAC?

I would like to ask, with ARP request I got to believe that the destination MAC address was in all situations a broadcast address. Yet I got across a wireshark capture: I just cannot understand the ...
6
votes
1answer
1k views

subnetting is incorrect in show ip route output

in one of the CCNA courses theres the following image taken from a routing table: I'm not very confident with my subnetting skills, so please confirm if I'm wrong: 172 is a class B network (/16). So ...
6
votes
1answer
1k views

Why can I ping IP addresses from 127.0.0.1 to 127.255.255.254?

I thought that the loopback IP address of my machine is 127.0.0.1. I do not understand why am I able to ping IP addresses until 127.255.255.254.

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