The United States and the Islamic Republic of Iran do not have diplomatic relations. Switzerland is the U.S. protecting power and provides limited consular services to U.S. citizens in Iran.
Iran has no embassy in Washington, D.C.
More information about the Islamic Republic of Iran is available from the Department of State and other sources, some of which are listed here:
CIA World Factbook Iran Page
U.S. Virtual Embassy
History of U.S. Relations With Iran
U.S. Census Bureau Foreign Trade Statistics
Office of Foreign Assets Control Sanctions Page
Library of Congress Country Studies
Travel Information