網路城邦
回本城市首頁 STORY OF CITY
市長:Story Sharing  副市長:
加入本城市推薦本城市加入我的最愛訂閱最新文章
udn城市不分類不分類【STORY OF CITY】城市/討論區/
討論區不分版 字體:
上一個討論主題 回文章列表 下一個討論主題
IPv6
2009/07/19 11:25 瀏覽721|回應0推薦0

Story Sharing
等級:3
留言加入好友

Internet Protocol version 6 (IPv6) is the next-generation Internet Layer protocol for packet-switched internetworks and the Internet. IPv4 is currently the dominant Internet Protocol version, and was the first to receive widespread use. In December 1998, the Internet Engineering Task Force (IETF) designated IPv6 as the successor to version 4 by the publication of a Standards Track specification, RFC 2460.





In December 2008, despite celebrating its 10th anniversary as a Standards Track protocol, IPv6 was only in its infancy in terms of general worldwide deployment. A recent study by Google indicates that penetration is still less than one percent of Internet traffic in any country. The leaders are Russia (0.76%), France (0.65%), Ukraine (0.64%), Norway (0.49%), and the United States (0.45%). Although Asia leads in terms of absolute deployment numbers, the relative penetration is smaller (e.g., China: 0.24%). IPv6 is implemented on all major operating systems in use in commercial, business, and home consumer environments. According to the study, Mac OS X leads in IPv6 penetration of 2.44%, followed by Linux (0.93%) and Windows Vista (0.32%).





IPv6 has a much larger address space than IPv4. This results from the use of a 128-bit address, where IPv4 uses only 32 bits. The new address space thus supports 2^128 (about 3.4?0^38) addresses. This expansion provides flexibility in allocating addresses and routing traffic and eliminates the need for network address translation (NAT). NAT gained widespread deployment as an effort to alleviate IPv4 address exhaustion.





IPv6 also implements new features that simplify aspects of address assignment (stateless address autoconfiguration) and network renumbering (prefix and router announcements) when changing Internet connectivity providers. The IPv6 subnet size has been standardized by fixing the size of the host identifier portion of an address to 64 bits to facilitate an automatic mechanism for forming the host identifier from Link Layer media addressing information (MAC address).





Network security is integrated into the design of the IPv6 architecture. Internet Protocol Security (IPsec) was originally developed for IPv6, but found widespread optional deployment first in IPv4 (into which it was back-engineered). The IPv6 specifications mandate IPsec implementation as a fundamental interoperability requirement.





The general requirements for implementing IPv6 on a network host are specified in RFC 4294.





Enzymes are generally globular proteins and range from just 62 amino acid residues in size, for the monomer of 4-oxalocrotonate tautomerase, to over 2,500 residues in the animal fatty acid synthase. A small number of RNA-based biological catalysts exist, with the most common being the ribosome; these are referred to as either RNA-enzymes or ribozymes. The activities of enzymes are determined by their three-dimensional structure. However, although structure does determine function, predicting a novel enzyme's activity just from its structure is a very difficult problem that has not yet been solved.





Most enzymes are much larger than the substrates they act on, and only a small portion of the enzyme (around 3? amino acids) is directly involved in catalysis. The region that contains these catalytic residues, binds the substrate, and then carries out the reaction is known as the active site. Enzymes can also contain sites that bind cofactors, which are needed for catalysis. Some enzymes also have binding sites for small molecules, which are often direct or indirect products or substrates of the reaction catalyzed. This binding can serve to increase or decrease the enzyme's activity, providing a means for feedback regulation.





Like all proteins, enzymes are made as long, linear chains of amino acids that fold to produce a three-dimensional product. Each unique amino acid sequence produces a specific structure, which has unique properties. Individual protein chains may sometimes group together to form a protein complex. Most enzymes can be denatured ?that is, unfolded and inactivated ?by heating or chemical denaturants, which disrupt the three-dimensional structure of the protein. Depending on the enzyme, denaturation may be reversible or irreversible.





Sometime around 6000 BCE a nomadic herding people settled into villages in the Mountainous region just west of the Indus River. There they grew barley and wheat using sickles with flint blades, and they lived in small houses built with adobe bricks. After 5000 BCE the climate in their region changed, bringing more rainfall, and apparently they were able to grow more food, for they grew in population. They began domesticating sheep, goats and cows and then water buffalo. Then after 4000 BCE they began to trade beads and shells with distant areas in central Asia and areas west of the Khyber Pass. And they began using bronze and working metals.





The climate changed again, bringing still more rainfall, and on the nearby plains, through which ran the Indus River, grew jungles inhabited by crocodiles, rhinoceros, tigers, buffalo and elephants. By around 2600 BCE, a civilization as grand as that in Mesopotamia and Egypt had begun on the Indus Plain and surrounding areas. By 2300 BCE this civilization had reached maturity and was trading with Mesopotamia. Seventy or more cities had been built, some of them upon buried old towns. There were cities from the foothills of the Himalayan Mountains to Malwan in the south. There was the city of Alamgirpur in the east and Sutkagen Dor by the Arabian Sea in the west.





One of these cities was Mohenjo-daro (Mohenjodaro), on the Indus river some 250 miles north of the Arabian Sea, and another city was Harappa, 350 miles to the north on a tributary river, the Ravi. Each of these two cities had populations as high as around 40,000. Each was constructed with manufactured, standardized, baked bricks. Shops lined the main streets of Mohenjo-daro and Harappa, and each city had a grand marketplace. Some houses were spacious and with a large enclosed yard. Each house was connected to a covered drainage system that was more sanitary than what had been created in West Asia. And Mohenjo-daro had a building with an underground furnace (a hypocaust) and dressing rooms, suggesting bathing was done in heated pools, as in modern day Hindu temples.





The people of Mohenjo-daro and Harappa shared a sophisticated system of weights and measures, using an arithmetic with decimals. Whether these written symbols were a part of a full-blown written language is a matter of controversy among scholars, some scholars pointing out that this and the brevity of grave site inscriptions and symbols on ritual objects are not evidence of a fully developed written language.





The people of Mohenjo-daro and Harappa mass-produced pottery with fine geometric designs as decoration, and they made figurines sensitively depicting their attitudes. They grew wheat, rice, mustard and sesame seeds, dates and cotton. And they had dogs, cats, camels, sheep, pigs, goats, water buffaloes, elephants and chickens.

回應 回應給此人 推薦文章 列印 加入我的文摘

引用
引用網址:https://city.udn.com/forum/trackback.jsp?no=62823&aid=3533206