Chapter 21 can actually be subdivided into two large sections. The first deals with the new heaven and new earth in verses 1-8, The second deals with the new Jerusalem in verses 9-26.
Vision three of Revelation contrasts the two rival cities, the prostitute Babylon and the bride New Jerusalem. In chapters 17–18 John foresaw the prostitute city’s doom. In chapters 19–20 the Lamb-Bridegroom’s wedding was announced. Now, at last, the third vision concludes with a brief scene of the holy bride city. At once John is ushered into his fourth and final vision (21:9–22:5). This time, he is shown the bride city in detail. The most wonderful part of the final vision, however, is his portrait of Jesus among his people throughout eternity. – Easley, K. H. (1998). Revelation (Vol. 12, p. 393). Nashville, TN: Broadman & Holman Publishers.