In practice, ether is most commonly used. It doesn't matter which of the three topologies your interface card has, you can use any of the three keywords. However, since Ethernet, FDDI, and token-ring all contain 6-byte hardware addresses in their protocol headers, the tcpdump filter language treats ether, fddi, and tr as synonyms. There are also fddi (fiber distributed data interface) and tr (token-ring) keywords that match the hardware addresses of NICs (Network Interface Cards). For example, to find all broadcast packets, which are packets destined for the hardware address ff:ff:ff:ff:ff:ff, use: If, instead of using IP addresses, you wish to capture packets based on the hardware address of the network card, you use the ether (short for Ethernet) modifier. In Ethereal Packet Sniffing, 2004 Hardware Addresses (MAC Addresses) IPv6 autoconfiguration is compatible with both types of MAC addresses. This allows for far more MAC addresses, compared with 48-bit addresses. The OUI is still 24 bits, but the serial number is 40 bits. The IEEE created the EUI-64 (Extended Unique Identifier) standard for 64-bit MAC addresses. This process continues until the serial numbers for that OUI have been exhausted. Any NIC with a MAC address that begins with 00:05:85 is a Juniper NIC. A List of registered OUIs is available at Organizations that manufacture NICs, such as Cisco, Juniper, HP, IBM, and many others, purchase 24-bit OUIs from the Institute of Electrical and Electronics Engineers (IEEE), Incorporated Registration Authority. They have two halves: the first 24 bits form the Organizationally Unique Identifier (OUI) and the last 24 bits form a serial number (formally called an extension identifier). Historically, MAC addresses were 48 bits long. There are real-world exceptions to this, often due to mistakes by NIC manufacturers, but hardware MAC addresses are considered unique on the exam. Commonly used ICMP types are echo request and echo reply (used for ping) and time to live exceeded in transit (used for traceroute).īurned-in MAC addresses should be unique. ICMP has no concept of ports, as TCP and UDP do, but instead uses types and codes. ICMP is the Internet Control Message Protocol, a helper protocol that assists Layer 3 (IP) by troubleshooting and reporting error conditions: Without ICMP, IP would fail when faced with problems like routing loops, ports, hosts, or downed networks. It is also used for query−response applications, such as DNS queries. It is commonly used for applications that are “lossy” (i.e., they can handle some packet loss), such as streaming audio and video. UDP is the User Datagram Protocol, a simpler and faster cousin of TCP. Most operating systems require super-user privileges to open a reserved port, but any user may open an (unused) ephemeral port. Reserved ports are 1023 or lower ephemeral ports are 1024 through 65535. The two types of ports are reserved and ephemeral. The TCP port field is 16 bits, allowing port numbers from 0 to 65535. TCP connects from a source port to a destination port-for example, source port 51178 and destination port 22. TCP can reorder segments that arrive out of order and retransmit missing segments. ![]() TCP is the Transmission Control Protocol, a reliable Layer 4 protocol that uses a three-way handshake to create reliable connections across a network. Hosts may also access IPv6 networks via IPv4 this is called tunneling. ![]() Systems may be “dual stack” and use both IPv4 and IPv6 simultaneously. ![]() Too few available IPv4 addresses in a world where humans (and their devices) outnumber them is a fundamental problem: This was one of the factors leading to the creation of IPv6, which uses much larger, 128-bit, addresses. A 32-bit address field allows 2 32, or nearly 4.3 billion, addresses. IPv4 uses 32-bit source and destination addresses, usually shown in “dotted quad” format (e.g., “192.168.2.4”). If connections or reliability are required, they must be provided by a higher-level protocol carried by IP, such as TCP. It is also connectionless and unreliable, providing “best effort” packet delivery. IPv4 is Internet Protocol version 4, commonly called “IP.” It is simple, designed to carry data across networks. Newer EUI-64 MAC addresses are 64 bits long. Historically, MAC addresses were 48 bits long, divided in halves: The first 24 bits represented the Organizationally Unique Identifier ( OUI) the last 24 bits, a serial number (formally called an extension identifier).
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